Abstract:
A printhead alignment sensor for an ink jet printer includes at least two terminals defining a gap therebetween. An electrical measuring device detects a change in an electrical parameter between two of the terminals when ink is in the gap between the at least two terminals.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an ink jet printer, and, more particularly, to a head-to-head alignment method and sensor for an ink jet printer. 
     2. Description of the Related Art 
     Many inkjet printers contain two printheads mounted to the same carrier. For example, one printhead can be monochrome only and the other printhead can be color only. Both printheads can be used on the same printed image. The monochrome printhead provides the saturated black and the color printhead provides all other colors. The dots fired by the two heads must be precisely aligned, horizontally and vertically, or else print quality defects will be seen. For example, the black and color dots will overlap and unprinted white areas will remain. 
     Vertical alignment errors cause vertical offsets between horizontal lines printed by each printhead. Horizontal alignment errors cause horizontal offsets between vertical lines printed by each printhead. 
     Many printers to date include a manual method of performing horizontal and vertical head-to-head alignment. Usually, this includes the printer driver printing a test page which includes a continuum of alignment possibilities, and having the user manually type-in at their personal computer a number or letter representing the pattern having the best alignment. From this input, the driver saves timing offsets to allow horizontal head-to-head alignment. Vertical alignment is achieved by moving the printed swath vertically within a printhead. A small percentage of the printhead nozzles are unused to allow the swath to be moved vertically. 
     What is needed in the art is an automatic, rather than manual, head-to-head alignment process, which removes the burden from the user. 
     SUMMARY OF THE INVENTION 
     The present invention provides a simple, low-cost, head-to-head alignment sensor and a simple, automatic head-to-head alignment method. 
     The invention comprises, in one form thereof, a printhead alignment sensor for an ink jet printer. At least two terminals define a gap therebetween. An electrical measuring device detects a change in an electrical resistance between two of the terminals when ink is in the gap between the at least two terminals. 
     The invention comprises, in another form thereof, a method of horizontally aligning a first printhead and a second printhead in an ink jet printer. A substrate having a target area with a width approximately equal to a width of an ink drop is provided. A carrier of the first printhead is moved from a first location toward the target area. A plurality of aligned first ink drops are jetted from the first printhead when the carrier of the first printhead is at a first jetting location. The aligned first ink drops are substantially parallel to the target area. It is sensed whether at least one of the first ink drops has been jetted onto the target area. The carrier of the first printhead is returned to the first location. The moving, jetting, sensing and returning steps are repeated until at least one of the first ink drops has been jetted onto the target area. The jetting steps are performed at various first jetting locations. A first reference location of the carrier of the first printhead is recorded. The first reference location is a location of the carrier of the first printhead when it is sensed that at least one of the first ink drops has been jetted onto the target area. A carrier of the second printhead is moved from a second location toward the target area. A plurality of aligned second ink drops are jetted from the second printhead when the carrier of the second printhead is at a second jetting location. The aligned second ink drops are substantially parallel to the target area. It is sensed whether at least one of the second ink drops has been jetted onto the target area. The carrier of the second printhead is returned to the second location. The moving, jetting, sensing and returning steps are repeated until at least one of the second ink drops has been jetted onto the target area. The jetting steps are performed at various second jetting locations. A second reference location of the carrier of the second printhead is recorded. The second reference location is a location of the carrier of the second printhead when it is sensed that at least one of the second ink drops has been jetted onto the target area. At least one offset is calculated based upon the first reference location and the second reference location. 
     An advantage of the present invention is that printhead-to-printhead alignment can be performed automatically, rather than manually. That is, alignment can be performed without printing a test page. No user interaction is required. The alignment may take place automatically as soon as a new printhead is identified as having been installed. 
     Another advantage is that the method allows high accuracy of alignment at little cost. The sensing circuit requires just a few low cost components. Also, the cost of the sensor is much less than that of a reflective, optical type sensor. 
     Yet another advantage is that only a rough alignment of the sensor in the printer is required for ease of printer manufacturing assembly. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is an overhead schematic view of one embodiment of a slotted sensor of the present invention; 
     FIG. 2 is an overhead schematic view of another embodiment of a slotted sensor of the present invention; 
     FIG. 3 is a schematic view of one embodiment of a sensing circuit in which the sensor of FIG. 1 can be incorporated; 
     FIG. 4 is a front, sectional, perspective view of an ink jet printer including the sensing circuit of FIG. 3; 
     FIG. 5 is an overhead schematic view of the slotted sensor of FIG. 1 with a column of dots printed to the right of the gap; 
     FIG. 6 is an overhead schematic view of the slotted sensor of FIG. 1, rotated  90  degrees and with a row of dots printed above the gap; 
     FIG. 7 is an overhead schematic view of another embodiment of a slotted sensor of the present invention; 
     FIG. 8 is an overhead schematic view of yet another embodiment of a slotted sensor of the present invention; 
     FIG. 9 is an overhead schematic view of a further embodiment of a slotted sensor of the present invention; 
     FIG. 10 is an exploded, perspective view of a still further embodiment of a slotted sensor of the present invention; 
     FIG. 11 is an exploded, perspective view of another embodiment of a slotted sensor of the present invention; 
     FIG. 12 is a perspective view of yet another embodiment of a slotted sensor of the present invention; 
     FIG. 13 is an exploded, perspective view of a further embodiment of a slotted sensor of the present invention; and 
     FIG. 14 is an overhead view of another embodiment of a slotted sensor of the present invention. 
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate one preferred embodiment of the invention, in one form, and such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
     DETAILED DESCRIPTION OF THE INVENTION 
     In FIG. 1 there is shown one embodiment of a slotted sensor  40  of the present invention, including two copper terminals  42 ,  44  on a mylar substrate  46 . Terminals  42 ,  44  are separated by a gap  48  having a width  50  of approximately {fraction (1/600)}-inch, which is approximately the width of an ink droplet  32 . Gap  48  can be formed by laser cutting. An ohmmeter  52  has leads  54 ,  56  connected to terminals  42 ,  44 , respectively, to measure the resistance therebetween. When no ink drops  32  are between terminals  42  and  44 , the resistance between terminals  42  and  44  is many hundreds of megohms. If a single column of ink dots  32  is printed from a printhead substantially into gap  48 , as illustrated in FIG. 1, the resistance between terminals  42 ,  44  drops into the range of approximately between 0.5 and 3 megohms. Printing this column of ink drops  32  even one print element (pel) off-center of gap  48  leaves the resistance between terminals  42 ,  44  at several hundred megohms. One pel is defined herein as the width of one ink droplet. Once printed in gap  48 , the ink evaporates within a few seconds (this is due to the conductive nature of the ink, and the heat generated by the ohmmeter current through it), and the resistance returns to several hundred megohms. Thus, slotted sensor  40  is re-usable, i.e., it may be used for several alignment print passes. 
     Sensor  40  can be rotated 90 degrees in order to sense a horizontal row of ink dots instead of a vertical column of ink dots. Thus, two different sensors could be used, one sensor sensing a vertical column of ink dots aligned in the paper feed direction and another sensor sensing a horizontal row of ink dots aligned in the scan direction. The two sensors could be combined into a single sensor  140  (FIG. 2) including terminals  142 ,  144  separated by an L-shaped gap  146  having a width  148  of approximately {fraction (1/600)}-inch. Thus, sensor  140  can sense both horizontal rows of ink dots and vertical columns of ink dots. Gap  146  has a horizontal section  186  oriented in a scan direction of a printhead, and a vertical section  188  oriented in a paper feed direction of the printer. 
     Slotted sensor  40  can be incorporated in a sensing circuit  58 , as shown in FIG.  3 . The resistance of sensor  40  is used in a resistor divider in a comparator circuit such that its change from several hundred megohms to just a few megohms causes the output of comparator  60  to go high. This output is fed to the printer application specific integrated circuit (ASIC)  62  to indicate that the printed dot column has been printed in gap  48  of sensor  40 . 
     One embodiment of the horizontal head-to-head alignment method of the present invention includes positioning sensor  40  in the horizontal print path of carrier  30  (FIG.  4 ), in an approximate position specified in software. This approximate position of sensor  40  within an ink jet printer  64  is typically known to perhaps ⅛-inch. 
     In a next step of the method, carrier  30  moves leftward, and printer  64 , using a first printhead  34 , prints a single-pel-wide column of dots  32  somewhat to the right of sensor gap  48 , as shown in FIG.  5 . The column of dots can be printed just to the right of the left edge of terminal  44 , perhaps several pels away from gap  48 , but in an amount that is known to ensure that the column will be positioned to the right of gap  48 . Carrier  30  is then returned to the far right. 
     With carrier  30  again moving leftward, printer  64 , using the first printhead  34 , prints a single-pel-wide column of dots one pel further to the left than the previous column. Sensor  40  is monitored by ohmmeter  52  to determine whether the column is printed in gap  48 , or on the left edge of terminal  44 . If not, carrier  30  is returned to the far right and the above procedure is repeated such that increasingly leftward columns of dots are printed until gap  48  or the left edge of terminal  44  is located. If gap  48  or the left edge of terminal  44  is not located within a maximum number of tries, a dead sensor or other error is indicated. 
     Once gap  48  has been located, a known encoder position is recorded as the position carrier  30  must be in to print within sensor gap  48  with the first printhead  34 . Carrier  30  is then returned to the far right position. 
     In a next step of the method, carrier  30  moves leftward, and printer  64 , using a second printhead  34 , prints a single-pel-wide column of dots  32  somewhat to the right of sensor gap  48 , as shown in FIG.  5 . The column of dots can be printed just to the right of the left edge of terminal  44 , perhaps several pels away from gap  48 , but in an amount that is known to ensure that the column will be positioned to the right of gap  48 . Carrier  30  is then returned to the far right. 
     With carrier  30  again moving leftward, printer  64 , using second printhead  34 , prints a single-pel-wide column of dots one pel further to the left than the previous column. Sensor  40  is monitored by ohmmeter  52  to determine whether the column is printed in gap  48 , or on the left edge of terminal  44 . If not, carrier  30  is returned to the far right and the above procedure is repeated such that increasingly leftward columns of dots are printed until gap  48  or the left edge of terminal  44  is located. If gap  48  or the left edge of terminal  44  is not located within a maximum number of tries, a dead sensor or other error is indicated. 
     Once gap  48  has been located, a known encoder position is recorded as the position carrier  30  must be in to print within sensor gap  48  with the second printhead  34 . Offsets are calculated based on the encoder positions recorded for the first printhead  34  and the second printhead  34  and are used to correct subsequent print swaths. If the sensor is of the non-reusable type, separate sensors can be used for the first printhead and the second printhead. In this case, the separate sensors&#39; positions must be known to within a desired degree of tolerance. 
     One embodiment of the vertical head-to-head alignment method of the present invention includes positioning sensor  40  in the horizontal print path of carrier  30  (FIG.  4 ), in an approximate position specified in software. This approximate position of sensor  40  within an ink jet printer  64  is typically known to perhaps ⅛-inch. 
     A row of dots are printed on sensor  40  using first printhead  34 , at a y-direction coordinate (in the paper feed direction) that is known to be above the detecting area of sensor  40 , as shown in FIG.  6 . For many printheads, a row is printed by firing only one nozzle as the carrier is moved. 
     Another row of dots are then printed on sensor  40  using the first printhead  34 , at a y-direction coordinate one dot lower than the previous row. Sensor  40  is monitored by ohmmeter  52  to determine whether the row is substantially printed in gap  48 , or on the bottom edge of terminal  44 . If not, the above procedure is repeated such that increasingly downward rows of dots are printed until gap  48  or the bottom edge of terminal  44  is located. If gap  48  or the bottom edge of terminal  44  is not located with the lowest nozzle of the printhead, a dead sensor or other error is indicated. 
     Once gap  48  has been located, a known nozzle position, i.e., y-direction coordinate, is recorded as the position carrier  30  must be in to print within sensor gap  48  with the first printhead  34 . 
     In a next step of the method, printer  64 , using a second printhead  34 , prints a single-pel-high row of dots  32  somewhat above sensor gap  48 , as shown in FIG.  6 . The row of dots can be printed just above the bottom edge of terminal  44 , perhaps several pels away from gap  48 , but in an amount that is known to ensure that the row will be positioned above gap  48 . 
     Printer  64 , using second printhead  34 , then prints a single-pel-high row of dots one pel further downward than the previous row. Sensor  40  is monitored by ohmmeter  52  to determine whether the row is substantially printed in gap  48 , or on the bottom edge of terminal  44 . If not, the above procedure is repeated such that increasingly downward rows of dots are printed until gap  48  or the bottom edge of terminal  44  is located. If gap  48  or the bottom edge of terminal  44  is not located with the lowest nozzle of the printhead, a dead sensor or other error is indicated. 
     Once gap  48  has been located, a known nozzle position is recorded as the position carrier  30  must be in to print within sensor gap  48  with the second printhead  34 . Offsets are calculated based on the nozzle positions recorded for the first printhead  34  and the second printhead  34  and are used to correct subsequent print swaths. If the sensor is of the non-reusable type, separate sensors can be used for the first printhead and the second printhead. In this case, the separate sensor positions must be known within a desired tolerance. 
     A single-pel-width ink jet column print sensor can be formed in many ways. Each column sensor can be rotated  90  degrees and used as a row sensor, with a corresponding change in “x positions” to “y positions”. 
     In another embodiment, a non-reusable gap resistance sensor  66  (FIG. 7) has two or more gap positions. Each gap  68  is one pel wide and is separated from adjacent gaps  68  by a distance, for example, distance  70 , in an x-direction. Distance  70  is equal to an integer multiple of the width of a pel. Sensor  66  can be used in the orientation shown as a vertical column sensor. Alternatively, sensor  66  can be rotated  90  degrees and used as a horizontal row sensor. 
     In yet another embodiment, a sensor  150  (FIG. 8) is formed by adding an elongate terminal  152  above sensor  66 . A horizontal gap  154  between terminal  152  and sensor  66 , along with vertical gaps  68 , enables sensor  150  to detect both horizontal rows of ink dots and vertical columns of ink dots. 
     In yet another embodiment, a redundant sensor  72  (FIG. 9) operates similarly to sensor  40 . Terminal  74  includes a base  75  with tines  77  extending therefrom. Similarly, terminal  76  includes a base  79  with tines  81  extending therefrom. The resistance between terminals  74  and  76  is reduced when an ink dot column is aligned in a gap between tines  77  and  81 . Similarly, the resistance between terminals  74  and  76  is reduced when an ink dot row is aligned between base  75  and the distal ends of tines  81 , or between base  79  and the distal ends of tines  77 . Thus, like the sensors of FIGS. 2 and 8, sensor  72  of FIG. 9 can be used for both vertical and horizontal alignment. The method used in conjunction with sensor  72  is similar to that described above except that multiple columns are printed on each pass. 
     In a further embodiment of a vertical column detector (FIG.  10 ), an LED emitter  78  shines light through one-pel-wide transparent areas  80  in an opaque cover  82  via a light pipe  84 , and the light is sensed with a detector  86  mounted on a carrier  88 . A one-pel-wide column of ink drops is printed on cover  82  over an area  80 , blocking the light. When the light is blocked, the print position in the x-direction is known. Each area  80  is separated from adjacent areas  80  by an integer multiple number of pel widths. 
     In an embodiment of a horizontal row detector (FIG.  11 ), an LED emitter  156  shines light through a single one-pel-high transparent horizontal area  158  in an opaque cover  160  via a light pipe  162 , and the light is sensed with a detector  164  mounted on a carrier  166 . Dots are printed on a section of area  158 , and then carrier  166  is moved so that detector  164  is positioned over the section currently being used. 
     In another embodiment of a vertical column detector (FIG.  12 ), a black label  90  with one-pel-wide white bars  92  is sensed with a reflective sensor  94  mounted on a carrier  96 . A one-pel-wide column of ink drops is printed onto one of white bars  92 . When white is no longer sensed by sensor  94 , the print position of carrier  96  in the x-direction is known. 
     In another embodiment of a horizontal row detector (FIG.  13 ), ink dots are printed on a section of a single, horizontal, one-pel-high white bar  168  on a black label  170 , and a carrier  172  is moved so that a reflective sensor  174  is positioned over the section currently being used. When white is no longer sensed, the print position in the y-direction is known. 
     In another embodiment (FIG.  14 ), a one-pel-wide slot or opening  98  is provided in a platen  100  over a sensor  102 . Thus, platen  100  functions as a mask. Sensor  102  may be pressure sensitive, vibration sensitive, or a humidity sensor. When a one-pel-wide printed column of ink drops is printed through slot  98  and impinges upon sensor  102 , the print position in the x-direction is known. This detection device is reusable. 
     Cabling and connectors of the sensor of the primary embodiment of the present invention are simplified and cost-reduced as compared to an optical sensor because the sensor has only two terminals. The sensor base is small and can be made many-up with standard flex-cable manufacturing methods, then processed through a laser cut process to make the slot. 
     While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.