Patent Publication Number: US-7896460-B2

Title: Systems and methods for detecting intermittent, weak and missing jets with an inline linear array sensor

Description:
This is a Division of application Ser. No. 10/999,014 filed Nov. 30, 2004, now U.S. Pat. No. 7,427,118. The disclosure of the prior application is hereby incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     Some printers, such as direct marking office printers, have a plurality of nozzles. Each nozzle fires drops of ink during passes of printing operations. 
     To produce printed image of good quality, the nozzles need to fire jets with adequate ink drop sizes, with adequate strength, and without omission. 
     SUMMARY 
     When a printhead nozzle fires drops of an insufficient drop size, then the print density will be less than the neighboring jets and a streak will occur in the image. When a printhead nozzle does not consistently fire drops, then the missing drops of ink will also lead to smaller print density in the pixel columns that jet writes and thus streaks. When a printhead nozzle loses its ability to fire drops of ink, then there will be no ink written in the pixel columns addressed by that jet and thus streaks. When intermittent, weak or missing jets occur, it is desirable that such intermittent, weak and missing (IWM) jets be detected, and subsequent correction be made. 
     Systems and methods are provided for detecting intermittent, weak and missing jets with an inline linear array sensor. 
     In various embodiments of systems and method, a method for detecting intermittent, weak or missing jets comprises obtaining a test pattern having a plurality of dashes produced by a row of nozzles; obtaining a response profile based on sensor responses in a cross section of the test pattern; obtaining a metric for the response profile; and obtaining a difference between the metric and a reference. 
     These and other features and details are described in, or are apparent from, the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various exemplary details of systems and methods are described, with reference to the following figures, wherein: 
         FIG. 1  illustrates an exemplary printing failure; 
         FIG. 2  illustrates another exemplary printing failure; 
         FIG. 3  illustrates an embodiment of a test pattern; 
         FIG. 4  illustrates an embodiment of a sensed image; 
         FIG. 5  illustrates another embodiment of a sensed image; 
         FIG. 6  illustrates still another embodiment of a sensed image; 
         FIG. 7  illustrates embodiments of response profiles; 
         FIG. 8  illustrates embodiments of response profiles; 
         FIG. 9  is a flowchart outlining an embodiment of a method for detecting intermittent, weak or missing jets; and 
         FIG. 10  is a functional block diagram of an embodiment of a system for detecting intermittent, weak or missing jets. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       FIG. 1  shows an exemplary image  10 . As shown in  FIG. 1 , the process direction  12  runs in the vertical direction. The top of  FIG. 1  is the lead edge  15  of the image and the bottom of  FIG. 1  is the trail edge  17  of the image. The printer nozzles (not shown) are arranged in a series of rows in the cross process direction  14  and fire drops of ink as the receiving medium passes in the process direction  12  under the nozzles. 
     As shown in  FIG. 1 , the image  10  is printed by the printer nozzles over a plurality of passes  16 . In each pass  16 , the nozzles print a section of the image  10  in the process direction  12 . The width of the section may be one pixel. 
     During a pass  16 , the printhead (not shown) moves in the cross process direction  14  for one pixel. Accordingly, on the next pass of the media under the printhead a different pixel column will be written to. This process continues as the printhead continuously moves in the cross process direction  14 , and the image  10  is built by the sections produced in the passes  16 . 
     As shown in  FIG. 1 , the image  10  is printed with failed jets. In particular, the left portion  20  and the right portion  18  of the image  10  are printed with normal nozzles, while the center portion  22  of the image  10  is printed with a nozzle having a failure. 
     The cause of the failure is that the nozzle produces drops of ink with a smaller mass than the rest of the nozzles on the printhead. Thus, as shown in  FIG. 1 , although the center portion  22  of the image  10  is printed uniformly, the ink coverage is not as dense, because of the smaller ink drops. 
     In addition, because the drops have a smaller mass, they travel more slowly from the printhead to the medium than the drops of regular size. Thus, it took longer for the failed drops to cross the gap between the printhead and the medium on which the image is printed. Consequently, the center portion  22  of the image  10  is translated in the process direction  12  relative to the left portion  20  and the right portion  18  of the image  10 . In particular, as shown in  FIG. 1 , the center portion  22  of the image  10  is shifted down relative to the left portion  20  and right portion  18  of the image  10 . As a result, as shown in  FIG. 1 , the lead edge  15  of the image is not a straight horizontal line, but is shifted down in the middle portion where the weak nozzle prints. 
       FIG. 2  illustrates another exemplary image  30  having failed jets. In  FIG. 2 , the left portion  34  and the right portion  32  of the image  30  are printed with normal nozzles, while the center portion  36  of the image  30  is produced by nozzles having intermittent jets. Accordingly, the center portion  36  of the image  30  is not fully covered by ink, with uncovered areas where the jets were not fired. 
     However,  FIG. 2  does not indicate weak jets or jets with smaller drop mass. As shown in  FIG. 2 , the center portion  36  of the image  30  is not shifted or translated in the process direction  12 . The lead edge  35  of the solid pattern is a straight line in the cross process direction. 
     The failed jets may be detected and identified. In various exemplary embodiments, the failed jets may be detected at different points during a customer print job. For example, the failed jets maybe detected at the end of a job, at the end of a day, after a given number of prints, or on customer demand. 
     In various exemplary embodiments, the failure is detected using a test pattern. The dimensions of the test pattern may vary. The test pattern may be built up in multiple passes, depending on different requirements, such as the time available for detection, the techniques used in cleaning the test pattern, or a customer request. In various exemplary embodiments, a test pattern having only simple dashes produced during a single pass is used. In various exemplary embodiments, consideration is given whether the width of the linear array detector detector is greater than the process width, and whether all the nozzles can be printed and imaged by the linear array detector in a single pass. 
       FIG. 3  illustrates an embodiment of a test pattern  500 . In  FIG. 3 , the process direction  12  runs vertically from the top of the image to the bottom of the image. Only a section of the test pattern  500  in the cross process direction  14  (the horizontal direction) is shown in  FIG. 3 . 
     As shown in  FIG. 3 , the test pattern  500  includes an array of dashes  502 . The dashes  502  are far enough apart in the cross process direction so they can be distinguished by the linear array sensor. The dashes  502  are long enough in the process direction  12  so that 2 or more scans of the linear array detector occur while the dash passes under the linear array detector. 
     As shown in  FIG. 3 , the dashes  502  each extend in the process direction  12 . Each row of dashes is arranged in the cross process direction  14 . The dashes  502  in a row are spaced or separated from each other with a substantially equal distance in the cross process direction  14 . Each dash  502  is of substantially the same thickness and length. 
     In the test pattern  500  shown in  FIG. 3 , the nozzles of the printhead are spaced too closely to be distinguished if every nozzle prints dashes in a single row. Thus, as shown in  FIG. 3 , the odd nozzles print dashes in one row, and the even nozzles print dashes in another row. In particular, the dashes  504  printed by odd nozzles are in rows  1 - 12  (counting from the top), and the dashes  506  printed by the even nozzles are in rows  13 - 24 . 
     In  FIG. 3 , for each printhead, dashes are printed by the cyan, magenta, yellow, and black nozzles. In particular, the dashes in rows  1 - 3  and  13 - 15  are printed by the cyan nozzles, the dashes in rows  4 - 6  and  16 - 18  are printed by the magenta nozzles, the dashes in rows  7 - 9  and  19 - 21  are printed by the yellow nozzles, and the dashes in rows  10 - 12  and  22 - 24  are printed by the black nozzles. For each color strip, the dash is repeated 3times. For example, as shown in  FIG. 3 , a dash  508  from the odd cyan nozzles are printed 3 times, in rows  1 ,  2  and  3 . The more the dash is repeated, the higher precision with which measurements can be made. 
     When the process width  510  (the dimension in the cross process direction) is greater than the ink on drum detector width, only a subset of the nozzles can be monitored each time. The printheads may be moved so that all nozzles to be monitored are within the field of view of the ink on drum detector. In various exemplary embodiments, a control scheme is set up that monitors a different subset of the nozzles during each measurement iteration. In various other exemplary embodiments, the printheads are repositioned during the course of a single measurement. In such other exemplary embodiments, all nozzles print dashes, but the dashes are printed on different sections of the drum in the process direction. 
     A test pattern may be produced at the nominal imaging speed as the normal printing mode for direct marking printers. In some cases, the velocity of the imaging media is such that an image taken with the linear array sensor will be compressed in the process direction compared to the cross process direction. 
     When the test pattern is imaged at full drum velocity for a high speed printer, the dashes will need to be very long so that they can still be resolved after the compression in the cross process direction. Such a length requirement may exceed the area available for imaging on the drum, or may increase the amount of ink that is required for making measurements. 
     In various embodiments, the drum is slowed down when imaging a test pattern. Such a slow-down reduces the required ink amount. However, the speed need not be very slow. The minimal speed is constrained by the ability to maintain a uniform drum rotation motion quality. The maximum speed is constrained by the maximum length of the dashes that can be accommodated and ink usage. 
     In various exemplary embodiments, the linear array inline sensor is operated in diffuse mode or in specular mode. In diffuse mode, the detectors are oriented normal to the surface being imaged, and the illuminators are at some angle. The contrast arises from the difference in geometry between the ink and the substrate. The contrast also arises due to a difference between the reflectance of the substrate and the reflectance of the ink. In specular mode, the contrast arises because of the difference in the amount of light scattered when imaging the substrate and when imaging ink on the substrate. 
     In various exemplary embodiments, linear array inline sensors having separate red, blue and green illuminators are used. In such exemplary embodiments, largest signals are obtained by using complementary color to image the ink, as shown in  FIG. 4 . 
     In  FIG. 4 , a digital test pattern  520 , similar to the test pattern  500  shown in  FIG. 3 , is depicted on the left hand side. However, the dashes in  FIG. 4  have a width that is larger than their length. On the right hand side of  FIG. 4 , red illumination  540 , green illumination  560  and blue illumination  580  are depicted. The process direction  12  in  FIG. 4  runs in the vertical direction. There are 12 repeats of each dash for each color of ink. Dash group  522  (the first 12rows at the top of the test pattern) are written with cyan ink. Dash group  524  (the next 12 rows) are written with magenta ink. Dash group  526  (the following 12 rows) are written with yellow ink. Dash group  528  (the last 12 rows) are written with black ink. 
     As shown in  FIG. 4 , when the red illuminator  540  is used as a complementary color for cyan ink, there is a large contrast between the cyan ink and the drum substrate. Similarly, as shown in  FIG. 4 , the green illuminator  560  gives the largest contrast with magenta ink, and the blue illuminator  580  gives the largest contrast with yellow ink. For black ink, an illuminator of any color produces large contrast. 
     In various exemplary embodiments, each dash in the test pattern corresponds to one nozzle. The nozzle to which a dash corresponds to can be determined by counting the columns of dashes starting from one side of the image. 
     The presence of ink on the drum can either decrease or increase the response of sensors, depending on the relative contrast between the ink and the drum and the relative texture between the ink and the drum. For the ease of discussion, it is assumed that the presence of ink decreases sensor response. However, it should be appreciated that the discussion below also applies when the presence of ink increases sensor response. 
     In various exemplary embodiments, a projection of the imaged test pattern in the process direction  514  of sensor response is used to detect failed nozzles in a printed image. As shown in  FIG. 3 , the cross section  514  of the sensor response is a collection of profiles through the dashes  502  in the test pattern  500 . A profile may include sensor response along the cross process direction  14  at a particular location in the process direction  12 . In various exemplary embodiments, the cross section  514  is a collection of profiles through all the dashes  502  in a test pattern  500 . In various exemplary embodiments, the cross section  514  is a collection of profiles through part of the dashes  502  in a test pattern  500 . 
     In a response profile of the cross section  514  of sensor response, sensor response maxima occur at locations corresponding to positions where dashes do not exist, such as at the gaps  512  between dashes  502 . On the other hand, sensor response minima occur in the response profile at positions corresponding to locations where dashes  502  are printed. The positions of the minima are used to obtain the locations of the corresponding dashes. In various exemplary embodiments, the positions of the minima are also used to obtain information of the nozzles which produced the dashes. 
     In various exemplary embodiments, the centers of the dashes may be determined based on the cross section of sensor response, using the minima in the response profile. The determination may be achieved by any existing or later developed techniques. In various exemplary embodiments, the center of a dash line is determined based on an interpolation of the response data near the dash minimum, a mid-point of the interpolated left and right dash edge position where a reflection threshold is exceeded, a non-linear least squares fit to some average functional form of the dash, or a multi-dimension vector under Radar theory. 
     In  FIG. 3 , the cross section  514  may extend in the process direction  12 . Thus, a vertical strip of the image is sensed from the test pattern  500  in the vertical direction. As discussed above, because of the presence of the dashes  502 , this cross section  514  provides a generally Gaussian response profile, having low response at positions corresponding to the centers of the dashes  502 , and having high responses at locations corresponding to gaps  512  between the dashes. In various exemplary embodiments, the centers of the dashes are identified by identifying the minima in the response profile. In various exemplary embodiments, procedures are implemented for determining centers of the dashes even when the cross section is noisy due to noise from substrate structure and defective dashes. 
       FIG. 5  illustrates an image  600  sensed from a test pattern, such as the test pattern  500  shown in  FIG. 3 . In  FIG. 5 , the process direction  12  is in the vertical direction. The dark dashes  602  correspond to dashes in the test pattern. The sensed image shown in  FIG. 5  does not indicate significant failed jets. 
       FIG. 6  illustrates another sensed image  610  sensed from the same test pattern. Similar to  FIG. 5 , the dark dashes  602  correspond to dashes in the test pattern. As shown in  FIG. 6 , the dark dashes  602  of the middle column  604  of the sensed image  600  are shifted behind in the process direction  12 . In particular, each dash  602  in the middle column  604  appear lower than the corresponding dashes in the neighboring columns. Such a shift indicates that the nozzle that produced this column of dashes had weak jets, producing drops of decreased ink mass. As discussed above, an ink drop with reduced size has less velocity, travels slower, takes longer to cross the gap between the nozzle and the print medium, arrives at a later time to the medium which is moving, and thus appears lower on the medium in the process direction  12 , as shown in  FIG. 6 . 
     In  FIG. 6 , a cross section  606  of the sensed image  610  may be used to obtain a response profile. Different cross sections  606  may be applied along the cross process direction  14  at different positions along the process direction  12 . As discussed above, the presence of the dashes  602  reduces response strength. Thus, each response profile is generally a Gaussian curve. 
       FIG. 7  illustrates a plurality of generally Gaussian response profiles. Each profile represents the response in a cross section  606  of the image  610  in  FIG. 6  in the horizontal direction (cross process direction  14 ). Each cross section may be summed only with those linear array sensor scans that include a section of the test pattern dash. In various exemplary embodiments, various image processing steps are used to decrease the noise from background substrates. 
     In various exemplary embodiments, a plurality of metrics are extracted for each nozzle using the sensed image or sensed test pattern. The metrics may include the position of the drop in the cross process direction, such as the centers of the dashes in the cross process direction (x cen ); the position of the drop in the process direction, such as the centers of the dashes in the process direction (y cen ); and a metric related to the size of the drop, such as a minimal reflectance at the center of the drop (r min ). 
       FIG. 7  shows normalized response as a function of sensor pixel index in the cross process direction  14 . The sensor pixel index provides a coordinate that generally corresponds to the cross process direction  14 . As shown in  FIG. 7 , both the centers of the dashes in the cross process direction (x cen ) and the minimal reflectance at the center of the drop (r min ) can be determined. In particular, the position of a minimum in the Gaussian curve indicates the center of a dash. The magnitude of the response at the position of the minimum indicates the degree the drop attenuates the sensor response which in general is related to the drop size. 
     In  FIG. 7 , the solid line response profile  710  represents a cross section of the image in  FIG. 5  where there is no failed jets. Thus, the solid line response profile  710  substantially conforms to a series of Gaussian curves with substantially the same amplitude. The five minimal reflectance r min (i), where i=1, 2, 3, 4 and 5, have substantially the same magnitude. 
     On the other hand, the dashed line response profile  720  represents a cross section of the image in  FIG. 6 , where the dashes in the middle column  604  are shifted down in the process direction  12  (see  FIG. 6 ), due to failed jets of reduced ink drop size. Thus, because a dash that should have occupied a position in a test pattern may have been shifted out of the field of view of the linear array sensor, the reflectance from this position may be higher than expected. Accordingly, a cross section  606  that runs through a row of dashes in  FIG. 6  may contain higher reflectance at a position near the middle column  604 . 
     In particular, as shown in  FIG. 7 , the minimal response r min  ( 3 ) corresponding to the dashes of the middle column  604  in  FIG. 6  is significantly greater than the minimal reflectance r min  ( 1 ), r min  ( 2 ), r min  ( 4 ) and r min  ( 5 ) corresponding to the dashes in the neighboring columns in  FIG. 6 . Also, the minimal response r min  ( 3 ) is significantly greater than the minimal reflectance r min  ( 3 ) corresponding to the dashes in the middle column in  FIG. 5 . As discussed above, this is due to the shift of dashes in the middle column  604  of  FIG. 6  in the process direction  12 . Due to this shift, on average, the decrease of reflectance due to the presence of the dashes is reduced. Accordingly, the sensed reflectance is elevated. In various exemplary embodiments, this response elevation is used to detect weak jets with reduced ink drop size. Accordingly, the nozzle that produces the elevated response is flagged as a potential failed nozzle. 
     Referring back to  FIG. 6 , a cross section may also run along the process direction  12 . In particular, as shown in  FIG. 6 , the cross section  609  runs through the dashes  602  along the process direction  12 . The response profiles obtained from different cross sections  609  may be used to determine the position of the drop in the process direction, such as the centers of the dashes in the process direction (y cen ). 
       FIG. 8  illustrates a plurality of curves each representing a response profile of a cross section through the dashes  602  in  FIG. 6  in the process direction  12 . In  FIG. 8 , the response is shown as a function of the linear array sensor scan index in the process direction  12 . The linear array sensor scan index is proportional to a position along the drum in the process direction  12 . 
     In  FIG. 8 , the thin lines  802  correspond to dashes printed with nozzles having no failed jets. The thick line  804  corresponds to the dashes that are produced with failed jets and are shifted behind in the process direction  12 . In various exemplary embodiments, the difference between the thick line and the thin line is used to detect failed jets. In particular, the difference between the positions  806  of the minimal reflectance of the thin lines  802  and the positions  808  of the minimal reflectance of the thick line  804  is used to determine the centers of the dashes in the process direction (Y cen ). 
     In various exemplary embodiments, a number of signal processing techniques are used to extract the amount of the shift in the process direction  12  from the response profiles shown in  FIG. 8 . In various exemplary embodiments, the phase of the periodic response profiles is determined as a metric to detecting failed jets. The phase is proportional to the distance the dashes have been shifted in the process direction. In various exemplary embodiments, this phase is used to determine the position of the drop in the process direction (y cen ). 
     In various exemplary embodiments, a number of signal processing techniques are used to extract the amplitude of the response profiles shown in  FIG. 8 . The amplitude of the response profiles comprises an alternative metrics to the minimum reflectance. For a normally functioning jet with a low minimum reflectance, the amplitude of the response profile will be large. For a poorly functioning jet with a higher minimum reflectance, the amplitude of the response profile will be small. Similarly, in various exemplary embodiments, a number of signal processing techniques are used to extract the amplitude of the response profiles shown in  FIG. 7 . 
     In various exemplary embodiments, a threshold is established as a criteria for flagging a failed jet. The threshold allows for certain noise level in determining the process direction position of each jet, but is large enough to ensure that noise in the measurement of the process direction position is below this threshold. In various exemplary embodiments, the cutoff is chosen between two to three times the standard deviation of the noise in the measurement of a jets offset in the process direction. 
     In various exemplary embodiments, multiple criteria are established for identifying failed jets. For example, when either of the position of the drop in the cross process direction, the position of the drop in the process direction and the metric related to the size of the drop changes too much away from their expected values, a failed jet will be flagged. 
     In various exemplary embodiments, the position and magnitude of the drop is compared not to the mean position and magnitude of drops across the printhead, but instead to historical values of that drops position and magnitude before a potential failure. For example, even for a normal functioning printhead, there may still be some jet-to-jet variation in the position of the drop in the process direction. In various exemplary embodiments, a table may be built up of expected values for the position in the process direction, position in the cross process position, and drop magnitude for each nozzle. The table becomes more precise over time as more measurements are averaged together. Each subsequent measurement of position in the process direction, position in the cross process direction, and drop magnitude can be compared to the previously obtained values in the table. When a large variation from the expected value occurs, a failed jet is flagged. 
     The detected failed jets may be used for correction and adjustment. In various exemplary embodiment, the failed jets are detected when manufacturing the printheads. In various other exemplary embodiments, the failed jets are detected dynamically during printer operation. 
     In various exemplary embodiments, a flagged jet is verified in a verification step before the nozzle that produces the jet is purged. In various exemplary embodiments, the necessity of the verification step depends on the threshold chosen for flagging a jet as a failed jet. If the threshold is chosen too low and no verification step is used, then measurement noise on a normally operating nozzle may cause a purge. In various exemplary embodiments, a customers request is also taken into consideration when deciding whether a flagged jet needs to be verified before the associated nozzle is purged. 
       FIG. 9  is a flowchart outlining an embodiment of a method for detecting failed jets. As shown in  FIG. 9 , process of the method starts at step S 100  by initiating an intermittent, weak or missing jet monitor process. The process can be initiated when the machine is turned on and/or cycles up, at the end of a job, , after a given number of prints, or based on customer requests. Printing will resume after this monitor process is completed. 
     Next, in step S 200 , a test pattern or a target is printed. In various exemplary embodiments, the test pattern may be an array of dashes. Then, in step S 300 , the drum is slowed down. In various exemplary embodiments, the drum is slowed down to avoid having extremely long dashes in the test pattern which keeps the ink usage down to a minimum. 
     Next, in step S 400 , the image of the test pattern is captured by a sensor. In various exemplary embodiments, the sensor is a linear array sensor. Then, in step S 500 , the detected image is analyzed and metrics are extracted. The metrics include the cross process position, the process position, and the magnitude of a drop ejected from each nozzle in the field of view of the linear array sensor. Thereafter, process of the method continues to step S 600 . 
     In step S 600 , it is determined whether for any nozzle an extracted metric exceeds a threshold that indicates an intermittent, weak or missing jet. If it is determined at step S 600  that no metric exceeds a threshold, process of the method jumps to step S 1300 , where the monitor process ends and printing may be resumed. 
     On the other hand, if it is determined in step S 600  that a metric exceeds a threshold of an intermittent, weak or missing jet, process of the method continues to steps S 700 -S 1100  for confirmation. 
     In particular, in step S 700 , a verification target or a confirmation pattern is printed. In various exemplary embodiments, the confirmation pattern is identical to the test pattern, but printed on a different area of the drum. Such a confirmation pattern printed on a different area of the drum improves the accuracy of the failed jet detection in the monitor process, because the effect of some isolated point defect on the drum may be prevented from giving any false positive signal. In various other exemplary embodiments, the confirmation pattern may be a part of the test pattern that includes the suspected failed jets and a few jets adjacent to the suspected failed jets. Such a reduced size of the confirmation pattern in relation to the test pattern prevents doubling the amount of ink required each time for confirming failed jets. 
     Next, in step S 800 , the drum is slowed down. Then, in S 900 , the image of the confirmation pattern is captured by the sensor. Afterwards, in step S 1000 , metrics are extracted from the captured confirmation pattern. Process of the method then continues to step S 1100 . 
     In step S 1100 , it is determined whether the failed jets (the intermittent, weak or missing jets detected in steps S 200 -S 600 ) are confirmed. If the failed jets are not confirmed at step S 1100 , process of the method proceeds to step S 1300 , where operation of the method ends and printing is resumed. 
     On the other hand, if the failed jets are confirmed at step S 1100 , process of the method proceeds to step S 1200 , where the failed jets are wiped and/or purged. Operation then proceeds to step S 1250 . 
     In step S 1250 , a determination is made whether to perform more detection. If it is determined in step S 1250  to perform more detection, operation of the method returns to step S 200  to perform more detection, such as to detect whether the purge of the failed jets is effective, or to detect other failed jets. On the other hand, if it is determined in Step S 1250  that more detection is unnecessary, operation of the method continues from step S 1300 , where operation of the method ends and printing is resumed. 
     In various exemplary embodiments, steps S 700 -S 1100  in  FIG. 9  may be omitted. In such exemplary embodiments, the printhead is wiped and/or purged once it is determined in S 600  that a metric exceeds a threshold of an intermittent, weak or missing jet. 
       FIG. 10  is a functional block diagram of an embodiment of a system for detecting failed jets. As shown in  FIG. 10 , the system  100  may include an input/output (I/O) interface  110 , a controller  120 , a memory  130 , a data receiving circuit, routine or application  140 , a metrics extracting circuit, routine or application  150 , a failure detecting circuit, routine or application  160 , and a failure confirming circuit, routine or application  170 , each interconnected by one or more control and/or data buses and/or application programming interfaces  180 . 
     In various exemplary embodiments, the system  100  is implemented on a programmable general purpose computer. However, the system  100  can also be implemented on a special purpose computer, a programmed microprocessor or microcontroller and peripheral integrated circuit elements, an ASIC or other integrated circuits, a digital signal processor (DSP), a hard wired electronic or logic circuit, such as a discrete element circuit, a programmable logic device such as a PLD, PLA, FPGA or PAL, or the like. In general, any device capable of implementing a finite state machine that is in turn capable of implementing the flowchart shown in  FIG. 9  can be used to implement the system  100 . 
     The input/output interface  110  interacts with the outside of the system  100 . In various exemplary embodiments, the input/output interface  110  may receive input from the outside, such as the input  200 , via one or more links  210 . The input/output interface  110  may output data to output  300  via one or more links  310 . 
     The memory  130  may also store any data and/or program necessary for implementing the functions of the system  100 . The memory  130  can be implemented using any appropriate combination of alterable, volatile, or non-volatile memory or non-alterable or fixed memory. The alterable memory, whether volatile or non-volatile, can be implemented using any one or more of static or dynamic RAM, a floppy disk and a disk drive, a writable or rewritable optical disk and disk drive, a hard drive, flash memory or the like. Similarly, the non-alterable or fixed memory can be implemented using any one or more of ROM, PROM, EPROM, EEPROM, an optical ROM disk, such as a CD-ROM or a DVD-ROM disk and disk drive or the like. 
     In the exemplary embodiment of the system  100  shown in  FIG. 10 , the data receiving circuit, routine or application  140 , under control of the controller  120 , receives data from the input  200  via the one or more links  210  and the input/output interface  110 . The data may be signal detected by a sensor from printed test pattern or printed confirmation pattern. 
     The metrics extracting circuit, routine or application  150 , under control of the controller  120 , extracts metrics from the received data. The failure detecting circuit, routine or application  160 , under control of the controller  120 , determines whether a metric is greater than a threshold by comparing the difference between the metric with a reference. A failed jet is detected when the metric is greater than the threshold. 
     The failure confirming circuit, routine or application  170 , under control of the controller  120 , requests confirmation data, if a failed jet is determined by the failure detecting circuit, routine or application  160 . Consequently, under control of the controller  120 , the data receiving circuit, routine or application  140  receives confirmation data from a confirmation pattern. The metrics extracting circuit, routine or application  150  extracts metrics from the confirmation data. The failure detecting circuit, routine or application  160  detects failed jets from the metrics obtained from the confirmation pattern. 
     The failure confirming circuit, routine or application  170 , under control of the controller  120 , determines whether the failed jets are confirmed. The failure confirming circuit, routine or application  170 , after failed jets are confirmed, sends signal to output  300  via input/output interface  110  and the one or more links  310  for wiping and/or purging the printhead containing the nozzles associated with the detected failed jets. 
     The controller  120  may also instructs the data receiving circuit, routine or application  140 , the metrics extracting circuit, routine or application  150 , and the failure detecting circuit, routine or application  160  to perform more detection after the purging. 
     The data receiving circuit, routine or application  140 , the metrics extracting circuit, routine or application  150 , the failure detecting circuit, routine or application  160 , and the failure confirming circuit, routine or application  170  may receive data from, or send data to the memory  130 . In particular, the threshold or thresholds may be stored in memory  130  and may be updated as needed. 
     The method illustrated in  FIG. 9  may be implemented in a computer program product that can be executed on a computer. The computer program product may be a computer-readable recording medium on which a control program is recorded, or it may be a transmittable carrier wave in which the control program is embodied as a data signal. 
     While various details have been described, these details should be viewed as illustrative, and not limiting. Various modifications, substitutes, improvements or the like may be implemented within the spirit and scope of the foregoing disclosure.