Abstract:
A printhead alignment sensor for an ink jet printer includes two terminals defining a substantially linear gap therebetween. An ink support device supports ink in the gap between the terminals. An electrical measuring device detects a change in an electrical resistance between the terminals when ink is supported in the gap by the ink support device.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and apparatus for print alignment in an ink jet printer, and, more particularly, to a method and apparatus for bi-directional print alignment in an ink jet printer. 
     2. Description of the Related Art 
     When an ink jet printer prints the same horizontal print line (swath) in both left and right-going directions of the carrier, errors are induced due to the travel time of the ink droplets and cock of the carrier due to play in the carrier attachment. As illustrated in FIG. 1, the momentum of a left-going carrier  30  causes ink droplets  32  ejected by a printhead  34  to be carried leftward, resulting in a flight time error  36 . Similarly, the momentum of a right-going carrier  30  causes ink droplets  32  ejected by printhead  34  to be carried rightward, resulting in a flight time error  38  (FIG.  2 ). That is, without alignment, ejecting a vertical column of dots at a given physical encoder marking results in a printed column positioned to the left of the encoder marking location when the carrier is left-going, and results in a printed column positioned to the right of the encoder marking location when the carrier is right-going. In order to eliminate or reduce flight time errors, printers that feature bi-directional print modes must adjust print timing such that the columns of the above example converge on a single location. 
     Many printers include a manual method of doing “bi-directional alignment”. Usually, this involves 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 with best alignment. From this input, the driver saves timing offsets that allow left and right-going print to align properly. 
     Automatic bi-directional alignment methods have been featured in a few recent photo-quality ink-jet printers and plotters. Known methods of automatic bi-directional alignment are expensive and include a printed test pattern page scanned by an optical sensor residing on the carrier. 
     What is needed in the art is a low-cost, simplified bi-directional alignment sensor and, more generally, a simplified bi-directional alignment method. 
     SUMMARY OF THE INVENTION 
     The present invention provides a low-cost, simple sensor and method for performing bi-directional alignment in an ink jet printer. 
     The invention comprises, in one form thereof, a printhead alignment sensor for an ink jet printer including two terminals defining a substantially linear gap therebetween. An ink support device supports ink in the gap between the terminals. An electrical measuring device detects a change in an electrical resistance between the terminals when ink is supported in the gap by the ink support device. 
     The invention comprises, in another form thereof, a method of bi-directionally aligning a printhead in an ink jet printer. A substrate is provided having a target area with a width approximately equal to a width of an ink drop. A carrier of the printhead is moved in a first scan direction from a first location toward the target area. A plurality of aligned ink drops are jetted from the printhead when the carrier is at a first directional jetting location. The aligned ink drops are substantially parallel to the target area. Whether at least one of the ink drops has been jetted onto the target area is sensed. The carrier is returned to the first location. The moving, jetting, sensing and returning steps are repeated until at least one of the ink drops has been jetted onto the target area. Each first directional jetting location is closer to the target area than an immediately preceding first directional jetting location. A first reference jetting location of the carrier is recorded. The first reference jetting location is a location of the carrier when it is sensed that at least one of the ink drops has been jetted onto the target area while the carrier is moving in the first scan direction. The carrier is moved in a second scan direction from a second location toward the target area. The second scan direction is substantially opposite to the first scan direction. A plurality of aligned ink drops are jetted from the printhead when the carrier is at a second directional jetting location. The aligned ink drops are substantially parallel to the target area. Whether at least one of the ink drops has been jetted onto the target area is sensed. The carrier is returned to the second location. The second moving, jetting, sensing and returning steps are repeated until at least one of the ink drops has been jetted onto the target area. Each second directional jetting location is closer to the target area than an immediately preceding second directional jetting location. A second reference jetting location of the carrier is recorded. The second reference jetting location is a location of the carrier when it is sensed that at least one of the ink drops has been jetted onto the target area while the carrier is moving in the second scan direction. The first reference jetting location and the second reference jetting location are used to align ink jetted from the printhead when the carrier is moving in the first scan direction with ink jetted from the printhead when the carrier is moving in the second scan direction. 
     An advantage of the present invention is that the cost of the sensor is much less than that of a reflective, optical type sensor. The sensing circuit requires just a few low cost components, and the method allows high accuracy of alignment at little cost. 
     Another advantage is that the method requires only a rough alignment of the sensor in the printer for ease of printer manufacturing assembly. 
     Yet another advantage is that the method allows alignment to 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. 
    
    
     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 a schematic side view of a left-going printer carrier ejecting ink drops; 
     FIG. 2 is a schematic side view of a right-going printer carrier ejecting ink drops; 
     FIG. 3 is an overhead schematic view of one embodiment of a slotted sensor of the present invention; 
     FIG. 4 is a schematic view of one embodiment of a sensing circuit in which the sensor of FIG. 3 can be incorporated; 
     FIG. 5 is a front, sectional, perspective view of an ink jet printer including the sensing circuit of FIG. 4; 
     FIG. 6 is an overhead schematic view of the slotted sensor of FIG. 3 with a column of dots printed to the right of the gap; 
     FIG. 7 is an overhead schematic view of the slotted sensor of FIG. 3 with a column of dots printed to the left of the gap; 
     FIG. 8 is an overhead schematic view of another embodiment of a slotted sensor of the present invention; 
     FIG. 9 is an overhead schematic view of yet another embodiment of a slotted sensor of the present invention; 
     FIG. 10 is an exploded, perspective view of a further embodiment of a slotted sensor of the present invention; 
     FIG. 11 is a perspective view of a still further embodiment of a slotted sensor of the present invention; 
     FIG. 12 is an overhead view of another embodiment of a slotted sensor of the present invention; 
     FIG. 13 is a front, sectional, perspective view of an ink jet printer including the slotted sensor of FIG. 8; 
     FIG. 14 is an overhead view of yet another embodiment of a slotted sensor of the present invention; 
     FIG. 15 is an overhead view of the slotted sensor of FIG. 14 with a column of black ink drops printed thereon; 
     FIG. 16 is an enlarged, fragmentary, overhead view of the sensor of FIG. 15; 
     FIG. 17 is an overhead view of the slotted sensor of FIG. 14 with a column of color ink drops printed thereon; 
     FIG. 18 is an enlarged, fragmentary, overhead view of the sensor of FIG. 17; 
     FIG. 19 is a schematic, side view of one embodiment of a sensor positioning mechanism of the present invention in a first position; and 
     FIG. 20 is a schematic, side view of the sensor positioning mechanism of FIG. 19 in a second position. 
    
    
     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. 3 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 printhead  34  into gap  48 , as illustrated in FIG. 3, 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, and the resistance returns to several hundred megohms. Thus, slotted sensor  40  is reusable, i.e., it may be used for several alignment print passes. 
     Slotted sensor  40  can be incorporated in a sensing circuit  58 , as shown in FIG.  4 . 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 bi-directional alignment method of the present invention includes positioning sensor  40  in the horizontal print path of carrier  30 , in an approximate position specified in software. This approximate position of sensor  40  within an ink jet printer  64  (FIG. 5) is typically known to perhaps ⅛-inch. 
     In a next step of the method, carrier  30  moves leftward, and printer  64  prints a single-pel-wide column of dots  32  somewhat to the right of sensor gap  48 , as shown in FIG.  6 . 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  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 a left-going carrier  30  must be in to print within sensor gap  48 . Carrier  30  is then relocated to the far left position. With carrier  30  now moving rightward, printer  64  prints a single-pel-wide column of dots somewhat to the left of sensor gap  48 , as shown in FIG.  7 . The column of dots can be printed just to the left of the right edge of terminal  42 , perhaps several pels away from gap  48 , but in an amount that is known to ensure that the column will be positioned to the left of gap  48 . Carrier  30  is then returned to the far-left. 
     With carrier  30  again moving rightward, printer  64  prints a single-pel-wide column of dots one pel further to the right than the previous column. Sensor  40  is monitored by ohmmeter  52  to determine whether the column is printed in gap  48 , or on the right edge of terminal  42 . If not, carrier  30  is returned to the far-left and the above procedure is repeated such that increasingly rightward columns of dots are printed until gap  48  or the right edge of terminal  42  is located. If gap  48  or the right edge of terminal  42  is not located within a maximum numbers 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 a right-going carrier  30  must be in to print within sensor gap  48 . Offsets are then calculated based on the encoder positions recorded for left and right-going print and are used to correct subsequent print swaths. 
     The method above has been described with a left-going carrier printing to the right of the sensor gap, and a right-going carrier printing to the left of the sensor gap. However, it is to be understood that the present invention may include a left-going carrier printing to the left of the gap and then moving incrementally moving to the right to locate the gap. Similarly, a right-going carrier may print to the right of the gap and then move incrementally to the left to locate the gap. 
     The method described above is independent of the type of sensing device used. That is, given any sensor capable of denoting when a single-pel column of dots has been printed onto a given single-pel-wide print position or sensor edge, the above-described method may be used to perform bi-directional alignment. 
     In another embodiment, a non-reusable gap resistance sensor  66  (FIG. 8) has two or more gap positions. Each gap  68  is one pel wide and is separated from adjacent gaps  68  by a distance  70  in an x-direction. Distance  70  is equal to an integer multiple of the width of a pel. 
     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 a dot column is aligned in a gap therebetween. 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 (FIG.  10 ), an LED emitter  78  shines light through one-pel-wide areas  80  in a transparent 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 a still further embodiment (FIG.  11 ), 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 white bar  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 (FIG.  12 ), 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. 
     In yet another embodiment, an edge of a pressure sensor is suspended in a printable zone. A one-pel-wide column of ink drops is initially spit a distance away from the edge of the sensor. The column of ink drops is then spit closer to the sensor, in one-pel increments, until the ink starts to impinge upon the sensor edge. The edge of the sensor provides the needed known position in the x-direction. This embodiment is reusable and inexpensive. 
     In an alternative embodiment of a bi-directional alignment method, as shown in FIG. 13, the non-re-usable sensor  66  senses that a printed left-going, one-pel-wide column of ink drops has impinged upon a first fixed x position, and that a printed right-going, one-pel-wide column of ink drops has struck a second fixed x position. The first and second fixed x positions are separated from each other in the x-direction a known integer number of pel widths. In a first step of performing bi-directional alignment, the sensor is positioned in the horizontal print path of the carrier, in an approximate position specified in software. This approximate position is typically known to perhaps ⅛-inch, and can be the same as the position of sensor  40  in FIG.  5 . 
     With carrier  30  moving leftward, printer  104  (FIG. 13) prints a single-pel-wide column of dots somewhat to the right of the right-most sensor gap  68 , perhaps several pels away from right-most gap  68 , but in an amount that is known to ensure that the column will print to the right of right-most gap  68 . Carrier  30  is then returned to the far right. 
     With carrier  30  left-going, printer  104  prints a single-pel-wide column of dots one pel further to the left than the previous column. Sensor  66  is monitored to see if the column hits right-most gap  68 . 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 right-most gap  68  is located. If right-most gap  68  is not located within a maximum numbers of tries, a dead sensor or other error is indicated. 
     Once right-most gap  68  has been located, a known encoder position is recorded as the position a left-going carrier  30  must be in to print within sensor gap  68 . Carrier  30  is then relocated to the far left position. With carrier  30  now moving rightward, printer  104  prints a single-pel-wide column of dots somewhat to the left of the second right-most sensor gap  68 . The column of dots can be printed perhaps several pels away from second right-most gap  68 , but in an amount that is known to ensure that the column will be positioned to the left of second right-most gap  68 . Carrier  30  is then returned to the far-left. 
     With carrier  30  again moving rightward, printer  104  prints a single-pel-wide column of dots one pel further to the right than the previous column. Sensor  66  is monitored by ohmmeter  52  to determine whether the column is printed in second right-most gap  68 . If not, carrier  30  is returned to the far-left and the above procedure is repeated such that increasingly rightward columns of dots are printed until second right-most gap  68  is located. If second right-most gap  68  is not located within a maximum numbers of tries, a dead sensor or other error is indicated. 
     Once second right-most gap  68  has been located, a known encoder position is recorded as the position a right-going carrier  30  must be in to print within second right-most sensor gap  68 . Offsets are then calculated based on the encoder positions recorded for left and right-going print and are used to correct subsequent print swaths. 
     Cabling and connectors of the sensor 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. 
     Another embodiment of a slotted sensor  106  of the present invention is shown in FIG. 14. A gap  108  between terminals  110 ,  112  has alternating wider sections  114  and narrower section  116  to accommodate black ink drops and color ink drops, respectively. Terminals  110 ,  112  are mounted on and supported by substrate  118 . 
     Black ink has a greater dot size than does color ink (75 microns for black and 50 microns for the color), therefore when a swath of black ink that is one pel wide and 192 nozzles tall is printed versus a color swath that is one pel wide and 64 nozzles tall, inconclusive results may be obtained on the same gap. The results are inconclusive in that as the sensor is traversed in {fraction (1/1200)}-inch increments, a specific range of consecutive swaths for each ink (black and color) can be detected within the gap. For example, the black may have a range of thirteen consecutive print swaths that will trigger on one gap size, but if the gap size is increased, perhaps only five or six consecutive print swaths will trigger on the gap size. The same results can be obtained in the color as well, but a side effect occurs due to the increased gap size. The ink must be accumulated or built up to allow the signal to be seen. The increased gap size also decreases the signal strength. Another factor that the varying gap size affects is dry time, i.e., the time required for the sensor to return to an initial state. With an increase in the gap size, less time is required for the dots laid down to dry up and for the sensor to return to its initial state. The reason for this decrease in dry time is the fact that the same volume of water that is in the ink dots dries faster with an increase in surface area. In this case, a bigger gap size sensor has a greater surface area, thus it has a quicker dry time. This in turn reduces the time required to perform automatic alignment or provide quick and accurate results to the end user [customer], and uses less ink in the process. 
     Another benefit of the increased gap size is the number of times that an automatic alignment can be performed on a sensor before the sensor becomes useless. Every time a swath is printed in the gap region, a buildup of ink accumulates therein, which can increase the dry time and decrease the life span of the sensor. With the bigger gap, the same amount of build up can occur as in the smaller gap, but the effect is not as significant. 
     The bigger gap size is fine for the black ink dots, however it presents problems with the color ink dots. Since the color dots require a buildup to trigger the gap sensor, a sensor with a smaller gap is needed. The smaller gap allows the smaller color dots to trigger the sensor on the first swath pass that is printed within the gap because there is less area for the dots to cover. To further optimize the reliability and life of the gap sensor, the number of dots that is used for color and black can be changed. This change could be a decrease in dot count, or just a change in how the dots are positioned. A decrease in dot count allows a faster dry time and less buildup on the sensor, which increases the gap sensor&#39;s life. The positioning of the dots allows variability in the pattern used in the swath. 
     In a method of using gap sensor  106 , the position of sensor  106  is known and sensor  106  is placed in the print path of carrier  30 . For the black alignment, carrier  30  (traveling from the left to the right) prints a single-pel-wide column of dots just to the left of gap  108 . The printer then prints additional columns of dots in {fraction (1/1200)}-inch increasingly rightward increments until it reaches wider section  114  of gap  108 , as seen in FIGS. 15 and 16. Sensor  106  is thereby triggered. The position that first triggers sensor  106  is noted by the printer and is saved for later use in performing alignments. 
     Carrier  30  then travels from right to left and prints a single-pel-wide column of dots just to the right of gap  108 . The printer then prints additional columns of dots in {fraction (1/1200)}-inch increasingly leftward increments until it reaches wider section  114  of gap  108 , and sensor  106  is thereby triggered. The position that first triggers sensor  106  is again noted by the printer and is saved for later use in performing alignments. With the offsets, i.e., positions where the sensor is first triggered, being thus determined, the offsets can be used in an algorithm which aligns the black print head. For the color alignment, carrier  30 , traveling from the left to the right, prints a single-pel-wide column of dots just to the left of gap  108 . The printer then prints these columns of dots in {fraction (1/1200)}-inch increasingly rightward increments until it reaches narrower sections  116  of gap  108 , as shown in FIGS. 17 and 18. Sensor  106  is thereby triggered. The position that first triggers sensor  106  is noted by the printer and is saved for later use in performing alignments. 
     Carrier  30  then travels from right to left and prints a single-pel-wide column of dots just to the right of gap  108 . The printer then prints additional columns of dots in {fraction (1/1200)}-inch increasingly leftward increments until it reaches narrower section  116  of gap  108 , and sensor  106  is thereby triggered. The position that first triggers sensor  106  is again noted by the printer and is saved for later use in performing alignments. With the offsets, i.e., positions where the sensor is first triggered, being thus determined, the offsets can be used in an algorithm which aligns the color print heads. 
     An ink jet printer can include a sensor positioning mechanism  120  (FIG. 19) for moving a sensor of the present invention, such as sensor  40 , between a first position (FIG. 19) and a second position (FIG.  20 ). In the first position, the sensor is placed at the surface height of the paper in the paper path. In the second position, the sensor is placed below the level of a platen  122  so as to not interfere with the movement of paper  124  in direction  126  along the paper path. 
     Mechanism  120  includes a lever arm  128  that rotates about pivot  130 . A distal end of arm  128  has a slanted surface  132  and is attached to a sensor bed  134  for supporting sensor  40 . In the first position of FIG. 19, the distal end of arm  128  is biased, perhaps by a spring (not shown), through an opening  136  in platen  122  such that sensor  40  is at the vertical level of paper  124  in the paper path. This is the operational position of sensor  40 . 
     As a sheet of paper  124  proceeds along the paper path in direction  126 , a leading edge of paper sheet  124  engages slanted surface  132  of arm  128  and pushes arm  128  downward into the second position of FIG. 20, a bottom surface of paper  124  engaging a line of contact at the top edge of slanted surface  132 . The two opposite side edges of paper  124  are held taut in two respective nips between two respective pairs of rollers (not shown). The tautness of paper  124  overcomes the force of the spring and holds arm  128  in the second position. When paper  124  has moved beyond opening  136 , arm  128  is released by paper  124  and arm  128  returns to the first position of FIG.  19 . 
     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.