Abstract:
Methods and apparatus for extending the reliability and usefulness of a fullwidth printhead by providing a redundant temporary replacement printhead module that can be positioned to compensate for missing or faulty jet nozzles. In order to take advantage of a single extra printhead module and to be able to compensate for more than a single failed nozzle, the replacement module is mounted on a separate translating x-axis and preferably provided with roll adjustment along another axis so that an effective spacing of nozzles in the replacement module can be adjusted to align with detected defective nozzles. The fullwidth printhead is formed from at least one array of smaller printhead modules. The arrays may be offset by a non-integer spacing interval of the individual nozzles. For example, if the nozzle spacing is S, the offset may be S/2. By virtue of the x-translation and roll capabilities, a single replacement module can accommodate replacement of one or several defective nozzles spaced closer together than the total length L of the replacement module, even if the defective nozzle(s) are located on different printhead modules and have a non-integer spacing.

Description:
BACKGROUND  
       [0001]     The disclosure relates to methods and apparatus for extending the reliability and usefulness of a full width printhead by providing a redundant temporary replacement printhead module that can be positioned to compensate for missing or faulty jets.  
         [0002]     Printers using full width printheads (i.e., printbars) are known and offer several advantages over conventional printheads that must travel back and forth across a print medium to achieve printing of a page. Advantages include faster printing speed, quieter operation, improved reliability due to less moving parts, etc. However, full width printheads suffer from certain drawbacks.  
         [0003]     One particular drawback is a problem with defective nozzles. High productivity printers achieve enhanced productivity by employing a large number of nozzles. However, more nozzles result in greater opportunity for nozzle failure. For instance, a full width ink jet printhead array spanning a typical 8.5″ wide sheet of paper may have 7200 or more discrete individual jet nozzles, each of which must operate properly for the printer to produce a quality print. The problem is increased in high speed production architecture ink jet systems that can have a combined printhead width of up to 24″ or more.  
         [0004]     Partially due to manufacturing limitations and partially to reduce the cost of replacement, many pagewidth or full width printheads use a number of smaller replaceable printhead modules rather than a large single head. The printhead modules are either butted together to form a single linear array, or offset and staggered in length to provide full width functionality. Such full width printheads may also include multiple printhead modules arranged in series (but offset by a partial pixel width to achieve an effective increase in resolution of the head itself).  
         [0005]     For example, a prototype 24″ full color printer uses a first set of 32 modules (eight (8) three inch (3″) long 300 dpi staggered print modules for each of four colors C, Y, M, and K) to achieve 300 dpi printing. A second set of 32 printhead modules is offset by ½ pixel from the first set to effectively double the resolution of the printhead assembly to 600 dpi. Thus, 64 total printhead modules are present. This represents a total of 57,600 individual nozzles in the full width, full color printhead array. Having such a large number of individual jets increases the probability that any single ink jet will fail. This, coupled with very high printer usage in high speed production makes the probability and frequency of nozzle failure a significant problem.  
         [0006]     A simplified example of this is shown in  FIG. 1 , which represents a single color printbar  100  having a first set  200  of printhead modules  200 A-D and a second set  300  of printhead modules  300 A-D. The first set  200  includes individual modules  200 A-D that each contain a plurality of nozzles  210  spaced by a center-to-center distance S. The second set  300  similarly includes individual modules  300 A-D that each contain a plurality of nozzles  310  spaced by a distance S. However, the nozzles in the second set  300  are offset from the nozzles in the first set  200  by a spacing S/2. This effectively creates a composite array with twice the resolution (i.e., an effective spacing of S/2) of the individual printhead modules.  
         [0007]     In this simplified example, a defective nozzle  220  is present within printhead module  200 B. As is evident from the vertical lines, nozzles from the offset printhead module  300 B do not overlap with the single defective nozzle  220  shown. Accordingly, once at least one defective nozzle is present, the collective printbar  100  consisting of various printhead modules with nozzles is no longer capable of reproducing a complete image. Instead, the printbar  100  will print with a band or streak at the location of the defective nozzle where no printing can occur. Thus, once one or more nozzles become defective, image quality suffers.  
         [0008]     Failed ink jet detection systems are known in the art. Such technologies include, for example, drop sensors that recognize missing or misdirected drops. One such drop sensing device uses a light beam that is projected across the width of the printing medium and between the printhead and the printing medium to a detector. Based on the timing and degree of occlusion caused by an ink droplet passing through the light beam, the device can sense the size and directional accuracy of the ink droplets. A laser may also be provided for such detection. Examples of suitable detectors include U.S. Pat. No. 5,179,418, the subject matter of which is hereby incorporated herein by reference in its entirety, as well as Japanese Patent Publication No. 4-315914 and Japanese Patent Publication No. 4-276446.  
         [0009]     Even though nozzle failures, such as defective nozzle  220 , can be detected, no practical method exists to repair individual failed printheads, other than minor problems that can be fixed through routine cleaning or maintenance. Rather, typical repair requires a complete replacement of the printhead module containing one or more defective print nozzles. This, however, is problematic for at least three reasons. First, the failed printhead module is typically thrown away, which represents a significant investment in cost, even though only a single nozzle or jet may be defective. Second, a replacement printhead may not be readily available, which can increase printer down time. Third, typical replacement and necessary alignment must be performed by a qualified technician, which requires additional printer down time to schedule and complete the replacement. Particularly when the printer involved is used for high volume production runs, there is a very high cost associated with the necessity to stop the current production run and make such necessary printhead repairs.  
         [0010]     Various methods and attempts to improve the reliability of such printers are known, including for example, those disclosed in U.S. Pat. No. 5,581,284 to Hermanson, U.S. Pat. No. 6,089,693 to Drake et al., U.S. Pat. No. 6,462,764 to Kubelik, and U.S. Pat. No. 5,587,730 to Karz. Each of these four patents is commonly assigned to Xerox Corporation and hereby incorporated herein by reference in their entireties.  
       SUMMARY  
       [0011]     There is a need for a more cost-effective system to compensate for defective ink jets.  
         [0012]     There also is a need for a system and method that can extend the life of a printer before servicing or printhead replacement is necessary.  
         [0013]     There further is a need for a system and method that can enable compensation for defective ink jets on an array that includes nozzles with different alignment offsets using only a single replacement module.  
         [0014]     To provide redundancy at reduced cost, various exemplary embodiments provide one or more extra temporary replacement printhead modules in addition to the modules already provided to achieve fullwidth printing. These one or more replacement modules are not necessarily used during normal operation of the device and instead are mainly activated when one or more nozzles in the primary printhead modules are determined to be defective.  
         [0015]     If the circumstances are that the printhead module with a failure has more than one defective jet, this extra temporary spare module can obviously operate to replace two or more jets in the defective module.  
         [0016]     In order to take advantage of a single extra printhead module and to be able to compensate for more than a single failed jet, it is also possible that the module can be located to compensate for failed jets in two or more modules. If the modules are adjacent modules and the distance between failed jets is less than the length of the replacement printhead module, the module can be aligned to cover both defective jets. Alternatively, additional print passes could be added to compensate for more defective jets if they are not closely spaced.  
         [0017]     In order to further take advantage of a single extra printhead module and to be able to compensate for more than a single failed jet, it is also possible that the module can be provided with roll capability around at least one axis to compensate for failed jets in different modules having non-aligned or non-uniform spacing.  
         [0018]     In various exemplary embodiments, at least one extra printhead module is mounted on a separate translating x-axis or is otherwise adjustable along the x-axis. This architecture requires the addition of only a single printhead module because the x-axis translation ability allows alignment with any of the nozzles of the full width array.  
         [0019]     In various exemplary embodiments, one replacement module can be positioned to compensate for two or more missing jet nozzles. In a preferred embodiment, the replacement module can be rotated or rolled about one axis in addition to x-axis translation to align with one or more defective ink nozzles. This may be particularly useful when defective nozzles are on modules that are offset or otherwise non-aligned with other printbar modules.  
         [0020]     When one or more jets of a full width printhead is irreparably lost or otherwise defective, the jet(s) can be automatically detected by a suitable jet detector. An example of such known detection can be found, for example, in U.S. Pat. No. 6,089,693 to Drake et al. At this time, the printing process can be temporarily stopped and a spare temporary replacement printhead module moved into an x-direction position that covers the missing jet. The printing process can then be restarted and the printing of the image covered by the defective nozzle can be achieved using a nozzle from the spare replacement printhead module aligned with the defective nozzle. Conventional image processing techniques can provide the substituted drop by compensating the timing and placement of the replacement drop based on the known positional orientation of the spare replacement printhead module. This enables continued operation of the printer without the need for an extended stop to perform a complete replacement of a defective printhead module.  
         [0021]     Although printing could proceed indefinitely through use of the spare module, the defective printhead module may be replaced at an appropriate time, such as after completion of a production run or until service can be scheduled. At this time, it may not be necessary to purchase or install a new printhead module. Rather, because the temporary spare module only needs to have at least one jet that fires, the first time the “replacement” printhead module is used, it can itself be used to replace the defective printhead module having one or more defective nozzles. Then, the defective printhead module can be mounted as the new “replacement” temporary spare printhead module. This “replacement” module can theoretically be used for the life of the product, since it only needs to have one operational jet to serve its purpose as a temporary spare.  
         [0022]     The provision of more than one redundant print head module will further increase the average time between repairs of the printer. However, each added printhead adds additional cost and complexity. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]     Exemplary embodiments will be described with reference to the drawings, wherein:  
         [0024]      FIG. 1  illustrates an enlarged, schematic front view of a typical full width printbar made up of a plurality of individual printhead modules and including a defective jet nozzle;  
         [0025]      FIG. 2  illustrates a partially shown, isometric view of a full width array ink jet printer to which the printbar of  FIG. 1  can be provided;  
         [0026]      FIG. 3  illustrates a schematic circuit diagram of a control system for the printer of  FIG. 2 ;  
         [0027]      FIG. 4  illustrates an enlarged, schematic front view of a full width printbar further including a translating replacement printhead module in the printer of  FIG. 2 ;  
         [0028]      FIG. 5  illustrates a flow chart that achieves correction of a limited number of missing jets according to a first exemplary embodiment;  
         [0029]      FIG. 6  illustrates an enlarged, schematic front view of the printbar of  FIG. 4  showing a defective print nozzle after the replacement printhead module has been translated into a position to compensate for a single defective print nozzle;  
         [0030]      FIG. 7  illustrates an enlarged, schematic front view of the printbar of  FIG. 4  showing two defective print nozzles on adjacent print modules after the replacement module has been translated into a position that compensates for both defective print nozzles;  
         [0031]      FIG. 8  illustrates a flow chart that achieves connection of missing jets having non-aligned spacing according to a second exemplary embodiment; and  
         [0032]      FIG. 9  illustrates an enlarged, schematic front view of the printbar of  FIG. 4  showing two defective print nozzles on different offset print modules having dissimilar nozzle spacings. 
     
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
       [0033]     The disclosure is directed to compensating missing or defective elements in spot imaging reading and/or writing bars. In particular, it pertains to full width array raster input scanning (RIS) and raster output scanning (ROS) bars. These are formed from either a single full width array or more preferably a series of relatively short modules assembled together to have a requisite length and number of elements to scan or write an entire line of information with a high image resolution. In exemplary embodiments, the spot imaging bars are writing devices, preferably liquid ink jet printers, but can also include reading devices, such as LED bars.  
         [0034]     Exemplary printers can be of the continuous stream or drop-on-demand types, such as piezoelectric, acoustic, phase change wax-based or thermal, and have at least one printhead containing an array of nozzles from which droplets of ink are directed toward a medium, such as paper. The particular type of ink jet delivery methodology is not of particular concern, so long as temporary replacement printhead modules are compatible for use with any failed printhead modules in the main array.  
         [0035]      FIG. 2  illustrates an exemplary embodiment of an ink jet printer  8  including a pagewidth or large array black print bar  100  positioned to deposit ink on a curved recording medium placed on a rotating drum  11 , which is rotated by a multiple speed motor  9  and which rotates the drum  11  in the direction of an arrow  12  at selected different speeds. The print bar  100  has been assembled similar to that shown in  FIG. 1  to have a first set of staggered modules or printhead dies  200 A,  200 B, etc. and a second set of staggered modules or printhead dies  300 A,  300 B, etc. that are offset from the first set. The modules are assembled and aligned to form an extended width array as known in the art having a plurality of individual nozzles or jets. The nozzles are selectively energized to expel an ink droplet from the associated nozzle. The ink channels are coupled into a common ink manifold  14  mounted along and attached to the print bar  100  in sealed communication with the ink inlets of the channel dies through aligned openings. The manifold  14  is supplied with the appropriate ink, black in this example, from an ink container  16  through flexible tubing  18  attached thereto.  
         [0036]     In addition to the single color print bar  100  printing black ink, additional full width array printheads may be provided for printing a respective color, for instance cyan, magenta, and yellow. The appropriate ink can be supplied to the associated printhead by inclusion of an attached printhead ink tank coupled to the printheads themselves or by ink containers attached to the printheads through flexible tubing as used in connection with the black printbar. Alternatively, multicolor printheads could be utilized whereby two or more colors coexist within the same printhead. In this case, the alignment replacement jets on the replacement head would of course have to be of the same color as any given defective jet.  
         [0037]     To print an image, a controller  54  receives bit map images from a print driver which is either resident in the printer or is resident in an image generating device such as a personal computer, or a combination of the two. The bit mapped images are manipulated by the controller  54  such that the appropriate signals are transmitted to the printbar  100 . The drive signals generated by the controller  54  are conventionally applied via wire bonds to drive circuitry and logic on each of the printhead dies  200 A,  200 B,  300 A,  300 B, etc. of each printbar  100  (and any optional color printbars). Signals include pulsing signals that are applied to heat generating resistors or transducers formed in the heater dies or any other conventional or subsequently developed structure used by the printbar to eject ink from a select nozzle.  
         [0038]     The controller  54  may take the form of a microcomputer including a central processing unit, a read-only memory for storing complete programs and a random access memory. The controller  54  also controls other machine functions such as rotation of the drum  11  and movement of a positioning device  440  (to be described later in detail) associated with a carriage  410  to advance a temporary replacement printhead module  500  in position to compensate for missing or defective jets in the printbar  100 .  
         [0039]     Defects resulting from the failure of certain nozzles to eject ink during the printing process can generate images that are unacceptable. Such defects are considered a significant failure mode and can result in a user not printing with the printer until the non-printing nozzle is remedied either through a maintenance operation or by replacement of the printbar.  
         [0040]     In view of such printing defects, various embodiments provide a defective nozzle detection system  90  ( FIG. 3 ), and at least one replacement printhead module  500  mounted on a translating carriage assembly  410  used to compensate for one or more missing or defective jets. The defective nozzle detection system  90  detects which of the nozzles are not printing, and provides information that can be used by controller  54  to moving a properly functioning nozzle of the replacement printhead module  500  into alignment with the detected missing nozzle(s). The controller  54  also then sends image information to the properly functioning nozzle to fill in the missing image information from the defective nozzle(s), and prints the missing image information with the functioning nozzle(s).  
         [0041]     The carriage assembly  400  includes a translatable carriage  410  that is driven by lead screws  420  and  430  by a drive motor  440 . The carriage  410  includes curved frame members  450  and  460 , which support at least one temporary replacement printhead module  500 . Carriage  410  may include threaded apertures through which the lead screws  420  and  430  are threaded. The carriage  410  moves in the X-direction shown to traverse the printer parallel to the length of the printbar  100  and perpendicular to a direction of paper advancement. The replacement printhead module  500  can be conventional in construction and fabricated in accordance with the same techniques used to form individual print modules  100 A within printbar  100 . In preferred embodiments, replacement modules  500  are identical to modules  100 A and are thus suitable for eventual permanent replacement of malfunctioning printhead modules  100 A.  
         [0042]      FIG. 3  illustrates a printing system, including the printer  8 , which not only provides for determination of a missing nozzle, but which can also provide the capability of compensating for defective nozzles. The controller  54  is coupled to a bus  71  for transmission of image information and/or control signals between a plurality of printer devices and an image input device  74 . The image input device  74  includes a number of known image generators that generate image information in the form of various image description languages such as the known Page Description Language (PDL) and Postscript. The image input device could, for instance, include a personal computer, a computer workstation, a computer coupled to a scanner, or other known image input devices. The input image device  74  is coupled through a connecting bus to an interface  75  of the printer which provides for a compatible interchange of the image information generated by the image input device to the printer. The interface  75  is connected to the bus  71  and transmits image data and control data to the controller or to a Random Access Memory (RAM)  76  under the direction of the controller  54 . The printer, in addition, includes a Read Only Memory (ROM)  78  that includes sufficient memory for the storage of predetermined operating system or controlling programs such as is known by those skilled in the art. The controller  54  includes a plurality of circuits that enable the printer  8  to fill in missing data on a printed page, which occurs because of one or more defective nozzles.  
         [0043]     When a defective nozzle is discovered through use of defective nozzle detector  90 , a user may consider whether to continue printing. A user interface  80  may be provided, which typically appears on a display device, for instance, a cathode ray tube or liquid crystal display of the image input device  74 . The user interface may include the selection of two or more document resolutions. For instance, the user interface  80  may include a draft mode selector  82  and a high resolution mode selector  84  which, once selected, are transmitted to a resolution control circuit  85 . In addition, the user interface may include an image mend selector  86  that enables the user to select the option of filling in the missing data on a printed page due to one or more defective nozzle(s).  
         [0044]     Suitable selectors can include pushbuttons, touch sensitive screens, or mouse selectable items in menus as non-limiting examples. If the user does not select the image mend selector  86  function, the user can either decide not to print with the printer until the defective nozzle is corrected (i.e., stop current usage of the printer) or continue to print at reduced image quality (i.e., while retaining the current detected nozzle defect(s)). It is also possible to include a defective nozzle visual indicator  87  in the user interface. The indicator  87  indicates to the user that one or more defective nozzles are present. In various embodiments, the printing system may include a default setting where once a defective nozzle is identified, the system automatically enters the image mend mode  86  until otherwise changed by the user. This can allow near seamless automatic compensation for faulty jets.  
         [0045]     If the user selects the image mend selector  86  (or the feature is automatically enabled), then a signal responsive thereto is transmitted from the image input device  74  over the bus  71  to the controller  54 . Either prior to selection or in response thereto, defective nozzle detector  90  identifies which of the nozzles are defective. The defective nozzle detector  90  is incorporated as part of printbar control circuits  92 , which are coupled to the printbar  100 . In one example of a defective nozzle detector, the defective nozzle detector circuit detects when there is no current being carried by a particular drop ejector, which would indicate, for instance, an open heater or thermal transducer. It is also possible that other defective nozzle detection devices including ink sensing conductors placed within a channel could be used. In addition, a print of a diagnostic test pattern could be made to manually assess missing or faulty jets. The test pattern would allow the user to identify to the machine which of the nozzles are non-functioning. For instance, if the printer does not include nozzle detectors, the printbar could print a test pattern including nozzle identifiers, such as a number, which is printed by each of the functioning nozzles and which identifies a nozzle. The printer might print a test pattern responsive to a user selecting the image mend selector  86 . The missing number or numbers would indicate to the user which of the nozzles is non-functioning. The user would then input the nozzle number or numbers into the printer controller through, a user input device, such as a keypad  93 , of the user interface  80 .  
         [0046]     Once the defective nozzle(s) have been identified, the information is accessed by the controller  54  and is used by a nozzle control circuit  94 . The nozzle control circuit  94  provides a plurality of functions, which include enabling the storage of the identity of one or more defective nozzles as well as the direction of the storage of image data corresponding to a defective nozzle in a defective nozzle data RAM  96 . RAM  96  can be included in the RAM  76  or separately embodied. The nozzle control circuit  94 , upon receipt of the identity of the defective nozzle, would cause the defective data RAM to store appropriate data, which cannot be printed during printing of the image due to the defective nozzle. For instance, if there are two defective nozzles, then the image data, which is not printed by the first defective nozzle is stored in a plurality of registers  97 . This data, for example, corresponds to a single column of information wherein the image data for every pixel location of the column is stored for each of the lines of the missing column of the printed image. The second defective nozzle data is stored in a register  98 .  
         [0047]     During image processing, the controller  54  and the nozzle control circuit  94  transmits the stored image data from the RAM  96  to a suitable selected replacement nozzle for printing. The selected replacement nozzle could be determined as a function of the moving capabilities provided by the positioning device  440  or may be selected as a function of a distance measured in nozzle spacing from the defective nozzles. For instance, if a single nozzle is determined to be defective, the replacement printhead module  500  may be moved by a predetermined distance under control of the positioning device control circuit  95  of the controller  54 . The positioning device control circuit  95  transmits a signal representative of the desired nozzle spacing or the movement thereof to a printer control circuit  99  that is coupled to the positioning device  440 . After the positioning device control circuit  95  has transmitted a signal over the bus  71  to cause the positioning device  440  to move a predetermined distance from the defective nozzle, the controller  54  retrieves the defective nozzle data from the RAM  96  such that the data is printed by the replacement printhead module  500 .  
         [0048]      FIG. 4  is a schematic diagram of an exemplary printbar  100  that includes a defective nozzle  220  that has failed to eject ink along a pixel line that is parallel to the moving direction of the recording medium.  FIG. 4  also shows carriage assembly  400  and temporary replacement printhead module  500  at a non-use position, such as located at one extreme of the carriage assembly. In a single color (i.e., monochrome) application, only one such printbar  100  may be provided. In a multicolor printer, four such printbars may be needed, one for each of Cyan, Yellow, Magenta and blacK (C, Y, M, K). Additionally, to increase resolution, multiple arrays of each color may be provided in series, but laterally offset by a fraction of the individual nozzle spacing.  
         [0049]     In this particular example, printbar  100  is for a single color, such as black, and includes two serially oriented printbar arrays  200  and  300 . The first printbar array  200  has four individual printhead modules ( 200 A,  200 B,  200 C, and  200 D), each having a length L that collectively provide a full width array extending the length of the printer (e.g., 4×L) and have a uniform nozzle spacing of S. The second printbar array  300  also has four individual printhead modules ( 300 A,  300 B,  300 C, and  300 D) that collectively provide a full width array extending the length of the printer and has a uniform nozzle spacing of S. However, because second printbar  300  is offset from the first printbar  200  by a distance of S/2, the individual nozzles  210  of the first array do not overlap with corresponding nozzles  310  of the second array. Thus, for example, if the spacing S corresponds to 300 dpi (dots per inch), the combination of printbars  200  and  300  will result in an effective doubling of the resolution to  600  dpi. While shown to have four modules in each array, this is a non-limiting example. Any number of modules may be present in each array.  
         [0050]     Various methods of printer operation will be described with reference to  FIGS. 5-9 . A first method will be described with reference to  FIGS. 5-7 . The method starts at step S 5000  where the process advances to step S 5010  and image input data is received from a suitable source, such as from a scanner or electronic file. At step S 5020 , a routine to detect defective nozzle jets is performed. If no defective jets are detected, the process advances from step S 5030  to S 5040  where a normal print operation is performed. If, however, one or more defective jets are detected, the process advances to step S 5060  where it is determined if all of the defective jets are within a length L of the temporary printhead module. If not, the process advances to step S 5070  and displays a fault. At this point, the user may choose at step S 5080  to stop the current print job by proceeding to step S 5050 , or continue printing, albeit with defective nozzles, by proceeding to step S 5040 .  
         [0051]     If however, all defective jets are determined to be within length L in step S 5060 , the process advances to step S 5090  where the image data is extracted for the defective jet(s). Flow then advances to step S 5100  where the temporary replacement printhead module is positioned in line with the missing jet(s). Flow then advances to step S 5110  where the extracted data from the defective jet(s) is sent to predetermined nozzles of the replacement module. Flow then advances to step S 5120  where a temporary print mode operation is performed. During this temporary operation, properly functioning jets are printed as normal and the missing or defective jet(s) are printed by aligned nozzles in the temporary replacement printhead module through suitable timing and control of the image printing process. From step S 5120 , flow advances to step S 5050  where the process stops.  
         [0052]     Thus, during the method of  FIG. 5 , replacement printhead module  500  can be positioned as shown in  FIG. 6  to align a nozzle  510  of the replacement module with a defective nozzle  220  from the printbar  100 . Additionally, as shown in  FIG. 7 , if more than one defective nozzle is detected and the defective nozzles are within the width of the replacement printhead module  500 , the module  500  can be aligned to replace two or more defective jets, even if they exist on different printhead modules of the printbar. For example, here defective jets  220 A and  220 B from module  200 B and  200 C, respectively, are replaced by replacement nozzles  510 A and  510 B from replacement printhead module  500 .  
         [0053]     A second method of operation will be described with reference to  FIGS. 8-9 . In this example, the printbar  100  includes first array  200  and second array  300  that are offset by a spacing of S/2 as shown in  FIG. 9 . As also shown, one or more defective nozzles may be present on either array  200  or  300  (i.e., defective nozzles  220  and/or  320 ). Because of the possibility of failure of either of multiple offset or otherwise non-aligning or non-uniformly spaced nozzles, it may be desirable to provide two separate replacement modules, one capable of alignment with each printhead array spacing and orientation. However, this requires an extra redundant printhead module, which adds cost and processing complexity.  
         [0054]     This embodiment provides a mechanism in which a single replacement printhead module  500  can be used and aligned to either array  200  or  300  and may even be adjusted to compensate one or more defective jets on each of two non-aligned modules  200  and  300 . This is possible through a slight roll of the printhead module  500  about a roll axis in a direction R as shown. Thus, besides the ability to translate in the X-axis, the replacement module can be rotated slightly about a roll axis in direction R (which may be counterclockwise as shown or clockwise) to adjust the effective spacing between replacement nozzles to other than an even pitch spacing of S and to better align the replacement module with a defective jet.  
         [0055]     For example, in  FIG. 9 , two defective nozzles are shown (nozzle  220  on module  200 C and nozzle  320  on module  300 B that are separated in the X-axis by a spacing other than an integer multiple of S (i.e., 9S/2 in this example). Because of this, a translating nozzle array  500  having a common spacing S may be unable to accommodate compensation for one or the other defective nozzles in both modules. That is, due to movement constraints in the X-axis, alignment may be achieved with one module, but not necessarily the other because of the non-uniform or offset spacing. However, by allowing slight rotation of the module, replacement module  500  can be readily aligned with one or more defective nozzles, regardless of whether the defective nozzle is on the first array  200  or the second offset array  300 . In this example, module  500  can be aligned to have nozzles  510 A and  510 B that align with defective nozzles  320  and  220 , respectively.  
         [0056]     For example, when the replacement printhead module has nozzles arranged in a known sawtooth layout arranged on a diagonal, a nozzle spacing S that results in 300 dpi, and a module length of about 3 inches, a printhead module roll in axis R of at little as 10 m radians can cause a shift in spacing between a far left nozzle and a far right nozzle of a sawtooth of 42 μm, which corresponds to 1/600 dpi. In the illustrated example of a spacing S that corresponds to 300 dpi nozzle spacing, results in the ability to accommodate alignment with a spacing that is a multiple of S/2 as illustrated in  FIG. 9 . Similar results can be achieved using a straight nozzle array as shown in the simplified drawings. The roll would be similarly adjusted based on known geometric relationships to achieve a desired effective nozzle spacing between nozzles  510 A and  510 B.  
         [0057]     An example of a method of correction using this structure will be described with reference to  FIG. 8 . The method starts at step S 800  and advances to step S 8010  where image input data is received. From step S 8010 , flow advances to step S 8020  where defective nozzle jets are detected. If no defective jets are detected, the process advances from step S 8030  to S 8040  where a normal print operation is performed. If, however, one or more defective jets are detected, the process advances to step S 8060  where it is determined if all of the defective jets are within a length L of the temporary printhead. If not, the process advances to step S 8070  and displays a fault. At this point, the user may choose at step S 8080  to stop the current print job by proceeding to step S 8050 , or continue printing, albeit with defective nozzles, by proceeding to step S 8040 .  
         [0058]     If however, all defective jets are determined to be within length L in step S 8060 , the process advances to step S 8090  where the image data is extracted for the defective jet(s). Flow then advances to step S 8100  where the temporary replacement printhead module is positioned in line with the missing jet(s). Flow then advances to step S 8110  where it is determined whether all defective jets are aligned with corresponding nozzles of the replacement printhead module. If they are, flow advances to step S 8130  where the extracted data from the defective jets is sent to predetermined nozzles of the replacement module. Flow then advances to step S 8140  where a temporary print mode operation is performed. During this temporary operation, properly functioning jets are printed as normal and the missing or defective jets are printed by aligned nozzles in the temporary replacement printhead module through suitable timing and control of the image printing process. From step S 8140 , flow advances to step S 8050  where the process stops.  
         [0059]     However, if it is determined at step S 8110  that all defective jets are not aligned, flow advances to step S 8120  where replacement module  500  is slightly rotated about axis R until defective nozzle(s) are properly aligned with a replacement nozzle of module  500 . From step S 8120 , flow advances to step S 8130 . Alignment can be achieved through use of adjustment tables, known mathematical geometric relationships, or through automated or manual visual inspection and subsequent calibration or adjustment. Thus, with only a single replacement module having nozzles with a spacing S, defective nozzles on two different modules that have non-uniformly aligned nozzles can be compensated for.  
         [0060]     Although printing could proceed indefinitely through use of the spare replacement module  500 , the defective printhead module ( 200  or  300 ) may be replaced at an appropriate time, such as after completion of a production run or when service can be scheduled. At this time, it may not be necessary to purchase or install a new printhead module in the array. Rather, because the temporary spare module only needs to have at least one jet that fires, the first time the “replacement” printhead module  500  is used, it can itself be used to replace the defective printhead module ( 200  or  300 ) having one or more defective nozzles. Then, the defective printhead module ( 200  or  300 ) can be mounted as the new “replacement” temporary spare printhead module  500 . This “replacement” module can theoretically be used for the life of the product, since it only needs to have one operational jet to serve its purpose as a temporary spare. All that is required is knowledge of the location of defective jet(s) so that the replacement module can be suitably positioned to have an operable jet aligned with defective jet(s) in the main printbar array.  
         [0061]     While the various described circuits  85 ,  94 , and  95  have been identified as part of the controller  54 , these circuits can be separate from the controller. In addition, the controller  54  as well as the described circuits  85 ,  94 , and  95  can be embodied as hardware, software, or firmware. It is known and commonplace to program and execute imaging, printing, document, and/or paper handling control functions and logic with software instructions for conventional or general purpose microprocessors. This is taught by various prior patents and commercial products. Such programming or software may of course vary depending on the particular functions, software type, and microprocessor or other computer system utilized, but will be available to, or readily programmable without undue experimentation from, functional descriptions, such as those provided herein, or prior knowledge of functions which are conventional, together with general knowledge in the software and computer arts. That can include object oriented software development environments, such as C++. Alternatively, the disclosed system or method may be implemented partially or fully in hardware, using standard logic circuits or a single chip using VLSI designs.  
         [0062]     It will be appreciated that various of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also, various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.