Patent Publication Number: US-2002001013-A1

Title: Ink jet printer having staggered nozzle array for color printing

Description:
[0001] This application is related to commonly assigned copending U.S. application Ser. No. ______, filed ______ by Bauer, Majette, and Dangelo, entitled “AUTOMATIC MAINTENANCE SYSTEM FOR DROP APERTURE PLATE (OPTICS PROTECTION),” attorney docket number 1092314, incorporated herein by reference. 
    
    
     
       BACKGROUND OF THE INVENTION  
       [0002] The subject invention is generally directed to color thermal ink jet printers, and more particularly to apparatus and techniques for improving the print quality of multiple cartridge color thermal ink jet printers.  
       [0003] An ink jet printer forms a printed image by printing a pattern of individual dots at particular locations of an array defined for the printing medium. The locations are conveniently visualized as being small dots in a rectilinear array. The locations are sometimes “dot locations”, “dot positions”, or “pixels”. Thus, the printing operation can be viewed as the filling of a pattern of dot locations with dots of ink.  
       [0004] Ink jet printers print dots by ejecting very small drops of ink onto the print medium, and typically include a movable carriage that supports one or more printheads each having ink ejecting nozzles. The carriage traverses over the surface of the print medium, and the nozzles are controlled to eject drops of ink at appropriate times pursuant to command of a microcomputer or other controller, wherein the timing of the application of the ink drops is intended to correspond to the pattern of pixels of the image being printed.  
       [0005] Color thermal ink jet printers commonly employ a plurality of printheads, for example four, mounted in the print carriage to produce different colors. Each printhead contains ink of a different color, with the commonly used colors being cyan, magenta, yellow, and black. These base colors are produced by depositing a drop of the required color onto a dot location, while secondary or shaded colors are formed by depositing multiple drops of different base color inks onto the same dot location, with the overprinting of two or more base colors producing secondary colors according to well established optical principles.  
       [0006] Print quality is one of the most important considerations of competition in the color ink jet printer field. Since the image output of a color ink jet printer is formed of thousands of individual ink drops, the quality of the image is ultimately dependent upon the quality of each ink drop and the arrangement of the ink drops on the print medium. One source of print quality degradation is insufficient drying of a first deposition ink drop prior to deposit of an overlying second ink drop. A further source of print quality degradation is the lack of precise ink drop placement on the print medium.  
       SUMMARY OF THE INVENTION  
       [0007] It would therefore be an advantage to provide a multiple printhead color ink jet printer provides for drying of ink drops prior to application of any overlying ink drops.  
       [0008] The foregoing and other advantages are provided by the invention in a color thermal ink jet printer that includes a print carriage movable along a carriage scan axis, a plurality of non-black color producing ink jet printheads supported by the print carriage and offset relative to each other so that their nozzle arrays are non-overlapping along the media scan axis, whereby the nozzle arrays of said non-black ink jet printheads traverse non-overlapping regions as the carriage is scanned along the carriage scan axis. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0009] The advantages and features of the disclosed invention will readily be appreciated by persons skilled in the art from the following detailed description when read in conjunction with the drawing wherein:  
     [0010]FIG. 1 is a schematic top plan view of the major mechanical components of a multiple printhead color ink jet printer in accordance with the invention.  
     [0011]FIG. 2 is a schematic side elevational sectional view illustrating, for one of the printheads of the printer of FIG. 1, the relation between the downwardly facing ink jet nozzles and the print media of the color ink printer of FIG. 1.  
     [0012]FIG. 3 is a schematic plan view illustrating the staggered arrangement of the nozzle arrays of the printhead cartridges of the printer of FIG. 1.  
     [0013]FIG. 4 is a schematic perspective view illustrating the staggered walls of the printhead carriage that support the printhead cartridge retaining structures in an arrangement that provides for a reduced head carriage width.  
     [0014]FIG. 5 is a top plan view illustrating the affixation of the printhead cartridge retaining structures to the printhead carriage staggered support walls.  
     [0015]FIG. 6 is a schematic elevational sectional view illustrating the affixation of the outboard flange of a printhead retaining structure that is on the outside of the group of printhead retaining structures.  
     [0016]FIG. 7 is a schematic elevational sectional view illustrating the affixation of overlapping flanges of adjacently mounted printhead retaining structures.  
     [0017]FIG. 8 is a perspective view illustrating an assembly of the printer of FIG. 1 that includes a drop detector, an aperture plate, and maintenance components for cleaning and protecting the aperture plate.  
     [0018]FIG. 9 is an exploded perspective view illustrating the drop detector of the assembly of FIG. 8.  
     [0019]FIG. 10 is a side elevational view schematically illustrating the operation of a light bender assembly of the drop detector of FIG. 9.  
     [0020]FIG. 11 is side elevational view illustrating the cleaning brushes and the aperture plate enclosure of the assembly of FIG. 8.  
     [0021]FIG. 12 is a top plan view illustrating the relation of the aperture plate and the aperture plate maintenance components of the assembly of FIG. 8.  
     [0022]FIG. 13 is a top plan view of the aperture plate of the assembly of FIG. 8.  
     [0023]FIG. 14 is a side elevational view illustrating the location of the optical detection zones of the drop detector of FIG. 9 relative to the aperture plate utilized therewith. 
    
    
     DETAILED DESCRIPTION OF THE DISCLOSURE  
     [0024] In the following detailed description and in the several figures of the drawing, like elements are identified with like reference numerals.  
     [0025] The subject invention is preferably implemented in a heated printing environment such as disclosed in commonly assigned copending U.S. application Ser. No. ______, filed ______ by Richtsmeier, Russell, Medin, Bauer, Cundiff, and Glassett, entitled “HEATER BLOWER SYSTEM IN A COLOR INK-JET PRINTER,” attorney docket number 189404, incorporated herein by reference.  
     [0026] Referring now to FIGS. 1 and 2, set forth therein are a schematic top plan view and a schematic side elevational sectional view illustrating, by way of illustrative example, major mechanical components of a multiple printhead color ink jet printer employing the invention. The printer includes a movable carriage  51  mounted on guide rails  53 ,  55  for translational movement along the carriage scan axis (commonly called the Y-axis in the printer art). The carriage  51  is driven along the guide rails  53 ,  55  by an endless belt  57  which can be driven in a conventional manner, and an encoder module  58  on the carriage  51  senses a linear encoder strip  59  to detect position of the carriage  51  along the carriage scan axis, for example in accordance with conventional techniques.  
     [0027] The carriage  51  supports four printhead cartridge retaining chutes  91  located at the front of the carriage  51  for retaining removable first through fourth ink jet printhead cartridges C 1 , C 2 , C 3 , C 4  (sometimes called “pens,” “print cartridges,” or “cartridges”) which are externally substantially identical. The printhead cartridges C 1 , C 2 , C 3 , C 4  include downwardly facing nozzles for downwardly ejecting ink to a print medium  61  which lies on a support print screen  65  located below the printhead cartridges. As shown in FIG. 2 for one of the printhead cartridges, the print media  61  advances along the media scan axis from beneath a print roller  63  pursuant to rotational cooperation thereof with other appropriate rollers, for example as disclosed in the previously cited application entitled “HEATER BLOWER SYSTEM IN A COLOR INK-JET PRINTER”.  
     [0028] The media scan axis, shown for example in FIGS. 1, 2, and  3 , can be considered as being generally tangential to the print media surface that is below the nozzles of the printhead cartridges and orthogonal to the carriage scan axis. It is noted that the media scan axis is sometimes called the “vertical” axis, probably as a result of those printers having printing elements that printed on a portion of the print media that was vertical. Also, the carriage scan axis is sometimes called the “horizontal axis”.  
     [0029] The cartridge chutes  71 ,  72 ,  73 ,  74  are side by side along the carriage scan axis and each is offset relative to an immediately adjacent chute along the media scan axis such that the nozzle arrays of the cartridges C 1 , C 2 , C 3  supported by the cartridge chutes  71 ,  72 ,  73  are non-overlapping along the media scan axis, and the nozzle arrays of the cartridges C 2 , C 4  are closely aligned along the media scan axis, as more particularly shown in FIG. 3. The cartridges C 1 , C 2 , C 3  comprise non-black color printing cartridges for producing the base colors of cyan, magenta, and yellow commonly utilized in color printing, while the cartridge C 4  comprises a black printing cartridge. The staggered arrangement of the pen chutes  71 ,  72 ,  73 ,  74  is readily observed in the sectional top plan view of FIG. 5 which is discussed further herein relative to the structure of the cartridge chutes and their installation on the printhead carriage  51 . The amount of stagger or offset along the media axis between the cartridges if discussed more specifically below in conjunction with the spacing of the nozzles of the nozzle arrays.  
     [0030] Referring now to FIG. 3, schematically depicted therein is the arrangement of the nozzle arrays of the cartridges C 1 , C 2 , C 3 , C 4  as viewed from above the nozzles of the cartridges (i.e., the print media would be below the plane of the figure). Each nozzle array of the cartridges C 1 , C 2 , C 3 , C 4  includes an even number of nozzles arranged in two columns parallel to the media scan axis, wherein the nozzle columns are staggered relative to each other. By way of illustrative example, each nozzle array includes 50 nozzles which are numbered as 1 through 50, with the 50th nozzle being at the end of the nozzle array that is first encountered by the leading edge of a print medium as it is advanced in accordance with the media advance direction shown in FIG. 3 as well as in FIG. 2, by which the leading edge of an advancing print medium first encounters the nozzle array of the printhead cartridge C 3 , then the nozzle arrays the printhead cartridges C 2 , C 4 , and finally the nozzle array of the printhead cartridge C 1 . Print direction as shown in FIG. 3 is such that the cartridge C 4 , the black print cartridge, is the first cartridge to encounter the print media.  
     [0031] The distance along the media scan axis between diagonally adjacent nozzles of each nozzle array, as indicated by the distance P in FIG. 3 for the cartridge C 4 , is known as the nozzle pitch, and by way of example is equal to the resolution dot pitch of the desired dot row resolution (e.g., {fraction (1/300)} inch for 300 dpi). In use, the physical spacing between the columns of nozzles in a printhead cartridge is compensated by appropriate data shifts in the swath print data so that the two columns function as a single column of nozzles.  
     [0032] Pursuant to the non-overlapping stagger of the non-black printing cartridges C 1 , C 2 , C 3  along the media axis, the areas or bands traversed by each of the cyan, magenta and yellow nozzle arrays in each carriage scan are non-overlapping. In this manner, ink drops ejected by the non-black cartridges in a given carriage scan do not fall on top of drops ejected in the same carriage scan, and ink drops of the non-black colors are in separate bands in each pass. This allows ink drops to dry before the application of any overlying or adjacent drops of a different color on a subsequent carriage scan and avoids ink bleed due to mixing of liquid ink of different colors. The black cartridge C 4  does not need to be offset along the media axis relative to all of the non-black printing cartridges, since dot locations having black dots are not printed with dots of another color. However, as discussed below, the black cartridge should not be aligned with the yellow cartridge along the media axis. Stagger or offset of the cartridges along the media axis also helps to reduce cockle since ink is distributed over a larger area than if the cartridges were side by side in a line along the carriage scan axis.  
     [0033] The amount of offset or stagger along the media axis between nozzle arrays should be at least 2 nozzle pitches to insure sufficient separation between colors in each pass. Thus, each of the media axis offsets between C 1  and C 2 , between C 2  and C 3 , between C 3  and C 4 , and between C 4  and C 1  should be at least 2 nozzle pitches.  
     [0034] It is noted that for further control of paper cockle, ink bleed, and coalescence, the staggered cartridge arrangement can be utilized in conjunction with known multiple pass print masking, as for example disclosed in commonly assigned U.S. Pat. No. 4,963,992, issued Oct. 16, 1990, to Mark S. Hickman, for “PRINTING OF PIXEL LOCATIONS BY AN INK JET PRINTER USING MULTIPLE NOZZLES FOR EACH PIXEL OR PIXEL ROW,” incorporated herein by reference; and in commonly assigned U.S. Pat. No. 4,965,593, issued Oct. 23, 1990, to Mark S. Hickman, for “PRINT QUALITY OF DOT PRINTERS,” incorporated herein by reference.  
     [0035] The black printing cartridge C 4  can be aligned with the cyan or magenta cartridge, but not with the yellow cartridge, since it is desirable to separate black and yellow ink drops to avoid muddy yellow printed dots. Black and yellow are very different in brightness and any spray ink particles from the black cartridge that rewet yellow dots would cause muddy yellow dots. Thus, for the particular example wherein the cartridges C 1 , C 2 , C 3  comprise cyan, magenta, and yellow producing cartridges, respectively, identified in FIG. 3 by the designations C, M, and Y, for cyan, magenta, and yellow, the black producing cartridge C 4 , identified in FIG. 3 by the designation K for black, can be aligned with the magenta cartridge C 2  as shown in FIG. 3. It is noted that the black cartridge could be positioned in alignment with the cyan cartridge C 1 , as shown by a nozzle array C 4 ′ depicted in by broken lines in partial form, which would provide for even greater separation between the yellow dots and the black dots applied in each carriage scan.  
     [0036] Referring now to FIGS.  4 - 7 , the cartridge chutes  71 ,  72 ,  73 ,  74  are substantially identical and are secured to the printhead carriage  51  in a manner that provides for a reduced printhead carriage width. As shown for the particular instance of the cartridge chute  73 , each chute includes a front wall  81  and side walls  83  which are mirror images of each other. A rearwardly extending top bracket  85  is connected between the top portions of the side walls  83 , and can be utilized to support a cartridge retaining leaf spring clip  84 . Vertical flanges  87  extend outwardly at the rear terminal edges of the side walls  83 . Each flange  87  includes forwardly extending locating pins  89  and locating apertures  91  formed on the back side of the flange in alignment with the locating pins  89 . Horizontal flanges  93  extend outwardly from the lower edges of the top bracket  85 , and have locating recesses  95  formed therein. The distance between the locating recesses on each flange is approximately equal to the offset between adjacent ones of the chutes  71 - 74 . Appropriate stops are provided within each of the chutes  71 - 74  for cooperating with retaining leaf spring clips  84  to fixedly position respective cartridges C 1  through C 4 .  
     [0037] The chutes  71 - 74  are secured by fasteners  95  to or against respective pairs of mounting standoffs  101 ,  102 ;  201 ,  202 ;  301 ,  302 ;  401 ,  402  formed in a support member  100  of the carriage  51 . The standoffs of each standoff pair are coplanar and offset relative to any adjacent standoff pair by the amount of desired offset between the nozzle arrays of adjacent cartridges. The standoffs are of different widths to accommodate the overlap of the flanges of adjacent offset cartridge chutes. In particular, a wide standoff is provided for each flange that does not have an underlying flange of an adjacent chute. A narrow standoff is provided for each flange that overlaps a flange of an adjacent chute. Each wide standoff is for engagement against a flange of a chute, and therefore includes locating pins  89  for engaging corresponding locating recesses  91  of the flange. Thus, the standoffs  101 ,  102  for the chute  71  are wide and narrow, respectively; the standoffs  201 ,  202  for the chute  72  are wide and narrow, respectively; the standoffs  301 ,  302  for the chute  73  are both wide; and the standoffs  401 ,  402  for the chute  74  are narrow and wide, respectively. By offsetting the chutes  71 - 74  so that their mounting flanges overlap reduces the width of the carriage  51 , which in turn reduces the width of the printer which must be sufficiently wide to permit over-travel of the carriage to insure the printheads cover the full width of the widest print medium for which the printer is designed to accommodate.  
     [0038] As shown for the representative example of the chute  73 , each chute is secured against corresponding standoff pairs and any underlying flanges, with the locating recesses  91  engaged in locating pins of an underlying flanae of an adjacent chute or in locating pins  89  of a wide standoff  101 ,  201 ,  301 ,  302 , or  402 . Captured between the flanges of each chute and the underlying surfaces of corresponding standoff pairs and any underlying flanges are edges of respective flexible circuits  97  having contacts engageable by corresponding interconnect contacts on the back of the printhead cartridges installed in the chutes. Resilient pads  99  are located behind the flexible circuits in recesses formed in the respective walls between each of the standoff pairs to apply pressure against the back of the flexible circuits when the cartridges are engaged in the chutes. By way of illustrative example, each resilient pad  99  includes raised bumps  98  at locations that correspond to electrical contact points between a flexible circuit  97  and a cartridge engaged therewith.  
     [0039] The chutes  71 - 74  are further secured by fasteners  113  which are positioned such that the flanges  93  of adjacent chutes can be secured with a single fastener. This can be achieved as a result of spacing the two semicircular recesses on each flange  93  by the desired offset between adjacent cartridges.  
     [0040] Referring now to FIG. 8, a drop detector assembly is provided for use in determining the offsets between the nozzle arrays of the printhead cartridges C 1 , C 2 , C 3 , C 4 . The assembly is conveniently located to one side of the media printing area, as shown in FIG. 1, and generally includes a drop detector  200 , an overlying aperture plate  125  that is coplanar with the portion of the print medium  61  underlying the nozzle arrays of the cartridges C 1 , C 2 , C 3 , C 4 , an enclosure  135  for protectively enclosing the aperture plate  125  when not in use, and brushes  137  for cleaning the aperture plate as it is moved into the enclosure  135 .  
     [0041] Interpen offsets are determined pursuant to detection of ink drops that pass through the aperture plate as each of the cartridges fires ink drops at the aperture plate, while scanning as well as stationarily positioned, as for example disclosed in commonly assigned U.S. Pat. Nos. 4,922,268; 4,922,270; and 5,036,340, incorporated herein by reference.  
     [0042] Referring now to FIG. 9, the drop detector assembly  200  includes a plurality of substantially identical elongated light bender assemblies  119  which are side by side and parallel to each other in alignment with the media scan axis. Each light bender assembly  119  includes a light bending source prism  116  and a light bending sensor prism  118  which are fixedly spaced apart from each other by elongated support members  122  connected to the sides of the prisms  116 ,  118  and parallel to the longitudinal axis of the light bender assembly. Each prism includes a top surface  124 , an angled surface  126  at a 135 degree included angle relative to the top surface, and a bottom surface  128  beneath the angled surface  126  and parallel to the top surface  124 , such that the included angle between the angled surface  126  and the bottom surface  128  is 45 degrees. Each prism further includes an inwardly facing surface  132  that is orthogonal to the longitudinal axis of the light bender assembly.  
     [0043] Respective upwardly facing LEDs  115  are located adjacent the bottom surfaces  128  of the source prisms  116  and respective upwardly facing photodiodes  117  are located adjacent the bottom surfaces of the sensor prisms  118 .  
     [0044] As shown more particularly in FIG. 10, the source LED  115  associated with a particular light bender assembly  119  is controlled to provide source illumination that enters the bottom surface  128  of the source prism  116  and is reflected at the angled surface  126  of the source prism pursuant to internal reflection. The internally reflected illumination exits the inward facing surface  132  of the source prism  116 , travels along the open region between the supports  122 , and enters the inward facing surface  132  of the associated sensor prism  118 . The illumination that enters the sensor prism  118  is downwardly reflected at the angled surface  126  of the sensor prism pursuant to internal reflection, and the downwardly reflected illumination exits the bottom surface  128  of the sensor prism  118  and illuminates the photodiode  117  positioned adjacent the bottom surface of the sensor prism. The region between the inwardly facing surfaces of the source and sensor prism of a light bender assembly comprises an optical detection zone  134  for detecting the presence of ink drops, wherein the presence of an ink drop in the optical detection zone  134  of a light bender assembly is detected by reduced light sensed by the photodiode  117  of the light bender assembly.  
     [0045] By employing internal reflection to accomplish light bending, optical coatings are avoided and the source and sensor prisms and the support members can be advantageously manufactured as an integral structure by injection molding which provides for inexpensive parts that can have complex geometries that enhance ease of assembly.  
     [0046] The LEDs  115  and the photodiodes  117  are contained between an lower mount  111  and an upper mount  113  which further cooperates with a top cover  121  to secure the light bender assemblies  119 . The top cover  121  includes ink passage slots  121   a  which are respectively aligned with the respective optical detection zones of the light bender assemblies  119 . Ink passage slots  111   a,    113   a  are also formed in the lower and upper mounts  111 ,  113 , in alignment with the optical detection zones of the light benders, wherein the slots in the lower mount  113   a  extending downwardly through openings  123   a  in a printed circuit board  123  which supports the assembly comprising the lower and upper supports, the LEDs, the photodiodes, the light benders and the top cover.  
     [0047] Referring now to FIGS. 11 and 12, as well as previously referenced FIG. 8, the ink drop detector  200  is utilized with an aperture plate  125  that is supported at the rear portion of an elongated support plate  127  which is engaged in guides  129  located at the corners of the top cover  121  for sliding displacement thereon parallel to the longitudinal extent of the light bender assemblies  119 . When the support plate  127  is displaced forwardly to a forward position, as shown in FIGS. 8 and 11, the aperture plate  125  overlies the drop detector  200 . When the support plate  127  is displaced rearwardly to a rearward position so that the aperture plate  125  is behind the drop detector assembly  200 , the forward non-apertured portion of the support plate  127  overlies the drop detector (as shown in FIG. 1) and thereby protects the optical elements of the optical detector when not in use from ink, paper dust, and other potentially contaminating materials that may become airborne inside the printer. By way of illustrative example, the support plate is displaced by a stepper motor  139  that turns a gear spool  141  which in turn pulls and pushes a drive strap  143  connected to the forward end of the support plate  127 .  
     [0048] For protection of the aperture plate  125  from ink, paper dust and other potentially contaminating materials that might become airborne in the printer, a box like protective housing  135  is located behind and offset relative to the top cover  121  for containing the aperture plate  125  when the supporting plate  127  is displaced rearwardly into an opening in the housing that is adjacent the rear edge of the top cover  121 . A pair of cylindrical brushes  137  are located at the opening of the protective housing  135 , and are configured to clean ink from the openings in the aperture plate  125  as it is displaced into the protective housing  135  after being used for cartridge offset determination.  
     [0049] Pursuant to the arrangement of the protective housing  135 . the elongated support plate  127  and the cylindrical brushes  137 , the aperture plate  125  is controllably positioned over the optical detection zones of the light bender assemblies  119  when it is required for determination of the offsets between cartridges. When the aperture plate is no longer needed, it is moved between the brushes  137  and into the protective housing  135 . In conjunction with the storage of the aperture plate  125  in the protection housing  135 , the non-apertured front portion of the support plate  127  overlies the optical elements of the drop detector to prevent contamination thereof.  
     [0050]FIG. 13 is a detail top plan view of an example of an aperture plate  125  that includes two identical vernier aperture patterns on either side of an elongated central slot  133 . The verniers and the central slot are positioned in alignment with the top cover slots  121   a  overlying the optical detection zones  134  of the light bender assemblies  119  when the support plate  127  is in a forward position.  
     [0051] After the aperture plate  125  has been utilized for determination of offsets between the cartridges C 1 , C 2 , C 3 , C 4 , it can be cleaned of ink build up by firing ink drops at the edges of the apertures in the aperture plate, and then passing the aperture plate through the cleaning brushes a number of times. By way of illustrative example, 50 drops from each nozzle of the magenta and yellow cartridges C 2 , C 3  are applied to a first vernier. Then,  50  drops from each nozzle of the magenta and yellow cartridges are applied to both longitudinal edges of the central slot, or to only the slot edge utilized for edge detection in conjunction with offset determination. After firing of ink drops at the edge or edges of the center slot, ink drops are applied to the second vernier in the same manner as for the first vernier, or 50 drops from each nozzle of the cyan, magenta, and yellow cartridges are applied to the second vernier for the situation where more ink was applied to the second vernier in the course of offset determination. The aperture plate is then parked into the enclosure, unparked out of the enclosure, finally parked in the enclosure, for a total of 3 passes through the brushes.  
     [0052] As a result of the light bender assemblies and the upwardly facing LEDs and photodiodes, the optical detection zones  122  in which ink drops are detectable can be closer to the exit side of the aperture plate  125 , as shown in FIG. 14, in comparison to known optical drop detectors in which an optical detection zone is formed by an LED facing an opposing photodiode. By locating the optical detection zones  122  closer to the exit side of the aperture plate  125 , drop detection to be reliably performed at higher drop fire rates for the following reasons. When an ink drop leaves a nozzle, it separates in a primary drop and one or more smaller secondary drops. The velocity of the primary drop is greater than the velocities of the second drops, and the distance between the primary drop and the secondary drops increases with distance from the source nozzle. In order to avoid having a primary drop and the secondary drops of a preceding drop in the drop detection zone at the same time, drop fire rate must be sufficiently low such that a primary drop does not enter the detection zone while a secondary drop from a preceding drop is still in the detection zone. Since the distance between a primary drop and its secondary drops increases with distance from the nozzle, drop firing rate must decrease with increased distance of the detection zone from the nozzle. The capability for reliable drop detection at higher drop fire rates translates into reduced time for pen offset determination which is performed by procedures involving the firing and detection of ink drops.  
     [0053] Although the foregoing has been a description and illustration of specific embodiments of the invention, various modifications and changes thereto can be made by persons skilled in the art without departing from the scope and spirit of the invention as defined by the following claims