Abstract:
A method of detecting malfunctioning ones of a plurality of nozzles of a printhead in an ink jet printer includes providing a sensor having at least two terminals defining at least one gap therebetween. An attempt is made to jet ink from a first of the nozzles into the at least one gap. A resistance between at least two of the terminals is measured to determine whether the ink has been jetted into the at least one gap. The attempting and measuring steps are repeated for each remaining nozzle.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to ink jet printers, and, more particularly, to a method and apparatus for checking the operation of the nozzles in an ink jet printer. 
   2. Description of the Related Art 
   Ink jet printhead nozzles are prone to clogging due to dried ink or debris physically impeding the nozzle plate orifice, or due to electrical failure, such as non-functional heater resistors that have failed due to electrostatic discharge, manufacturing defect on the silicon chip, broken TAB bond or chip trace connections, etc. 
   Even though the printhead ships from the factory with all nozzles testing good, defects including those listed above can occur in shipping, installation, or use of the head. While a head with such a defect is generally still usable, the resultant print quality defects are readily apparent to the user in the form of white lines in the printed pages. This is both a nuisance and a very visible negative contributor to the user&#39;s perception of the printer&#39;s quality. 
   Some known printers include a means to sense whether the nozzle/heater resistors read proper resistance. If so, an assumption is made that that nozzle is functioning correctly. Other known printers include a means to print a pattern on the page, each nozzle forming a block or similar pattern in an isolated page position, and moving an optical sensor over the page to sense presence or absence of the printed block or pattern. If a nozzle block is sensed, that nozzle is known to be functional. 
   Other known printers include means to adjust the printing algorithm so as to account for missing nozzles having been sensed. For instance, a normal print pass might be made, then the paper might be shifted a number of pels, then a second print pass might be made, this time to print the dot positions that were “out” on the first pass. 
   The drawbacks of the known schemes are that they require fairly expensive circuitry and/or special optical sensors to be used. Also, some require that a test page be printed to determine missing nozzles. 
   What is needed in the art is a simple, low-cost method and apparatus for performing automatic missing nozzle detection for an ink jet printer. 
   SUMMARY OF THE INVENTION 
   The present invention provides a simple, low-cost sensor for sensing whether ink is being emitted from individual nozzles, so that automatic adjustment might be made in printing to compensate for malfunctioning nozzles. 
   The invention comprises, in one form thereof, a method of detecting malfunctioning ones of a plurality of nozzles of a printhead in an ink jet printer. A sensor has at least two terminals defining at least one gap therebetween. An attempt is made to jet ink from a first of the nozzles into the at least one gap. A resistance between at least two of the terminals is measured to determine whether the ink has been jetted into the at least one gap. The attempting and measuring steps are repeated for each remaining nozzle. 
   The invention comprises in another form thereof, a sensor for detecting malfunctioning printhead nozzles in an ink jet printer. The sensor includes at least two terminals defining 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. 
   An advantage of the present invention is that malfunctioning nozzles are detected and compensated for such that the malfunctioning nozzles are transparent to the user and quality perception remains high. 
   Another advantage 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. 
   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 an enlarged view of certain areas of the sensor of  FIG. 2 ; 
       FIG. 4  is a schematic view of one embodiment of a sensing circuit in which the sensor of  FIG. 2  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 enlarged view of certain areas of the sensor of  FIG. 2  with a row of ink dots printed thereacross; 
       FIG. 7  is an enlarged view of certain areas of the sensor of  FIG. 2  with rows of ink dots printed along certain segments of the gap; and 
       FIG. 8  is an enlarged view of certain areas of the sensor of  FIG. 2  with a row of ink dots printed within a certain segment of the gap. 
   

   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 
   Referring now to the drawings and particularly to  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 between {fraction (1/1200)}-inch and {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 drop  32  is printed from a printhead 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, and the resistance returns to several hundred megohms. Thus, slotted sensor  40  is re-usable, i.e., it may be used for several repetitions. 
   One embodiment of a missing nozzle sensor  190  ( FIG. 2 ) operates similarly to sensor  40 , but is modified to allow detection of missing nozzles in a printhead having a column of 300 nozzles, each spaced vertically one pel apart. Sensor  190  includes two conductive terminals  192 ,  194  separated by and defining a serpentine gap  196 . Terminals  192 ,  194  have respective contacts  198 ,  200  to which an ohmmeter may be connected. Each of terminals  192 ,  194  has a height  202  of approximately 0.75 inch. A distance  204  between a left edge  206  of terminals  192  and a right edge  208  of terminal  194  is approximately 3.6 inches. Gap  196  has eight substantially horizontal sections  210  joined by seven vertical sections  212 . A distance  214  between a top horizontal section  210  and a bottom horizontal section  210  is approximately 0.5 inch. 
   Although each of sections  210  is substantially horizontal, a close inspection reveals that each section  210  is angled slightly downward from left to right. This can be most easily seen by comparing sections  210  with horizontal reference line  216 .  FIG. 3  illustrates the reason for the left to right downward tilting of sections  210 . The left side of  FIG. 3  is an enlargement of area  218  of  FIG. 2 , while the right side of  FIG. 3  is an enlargement of area  220 . Each section  210  is formed of a series of forty interconnected horizontal segments  222 . Each short horizontal segment  222  has a length  224  of eighty pels, i.e., approximately 2 millimeters. Each segment  222  of gap  196  is one pel high and is displaced by one pel in the vertical direction from one or two adjacent segments  222 . Each of the forty segments  222  in a section  210  corresponds to a respective nozzle on the printhead. Eight sections  210  are provided to thereby cover the total of 320 nozzles. 
   Sensor  190  can be incorporated in a sensing circuit  225 , as shown in FIG.  4 . The resistance of sensor  190  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 ink has been jetted into gap  196  of sensor  190 . 
   In one embodiment of a method of detecting a missing nozzle, re-usable gap sensor  190  is used to sense that a printed single-pel-tall row of seventy ink dots has struck a fixed y-axis position. Sensor  190  is positioned in the horizontal print path of a printhead  34  of a carrier  30  (FIG.  5 ), in an approximate position specified in software, aligned to within a few pels tolerance. This approximate position of sensor  190  within an ink jet printer  226  is typically known to perhaps ⅛-inch. Printhead  34  has a plurality of nozzles  228  displaced from one another in the vertical (paper feed) direction  230 . One of nozzles  228  is visible in FIG.  5 . 
   Printer  226  prints a single-pel-high row of ink dots  232  ( FIG. 6 ) across sensor  190  with a first nozzle  228 , i.e., an uppermost, leading paper-edge nozzle  228 . Print row  232  need only be printed across the x-axis range of the section  210  whose y-axis range includes the y-axis position of the first nozzle  228 . After printing row  232 , the resistance of sensor  190  is monitored by sensor circuit  225 . If the uppermost nozzle is working properly, and actually prints row  232 , ASIC  62  reads a positive signal and logs the nozzle as “good” in nonvolatile random access memory (NVRAM)  234 . Printer  226  then pauses long enough for printed row  232  to evaporate and for the resistance of sensor  190  to return to its initial large value. 
   If the uppermost nozzle  228  is deemed to be non-firing, this fact is logged in memory  234 . The above procedure including attempting to print a horizontal row of dots, etc., is repeated for each one of the remaining nozzles individually until the first jetting nozzle is identified. In the embodiment described herein, it is assumed that the uppermost nozzle  228  is identified as a jetting nozzle. 
   Knowing that the uppermost nozzle  228  is a jetting nozzle, printer  226  then uses the uppermost nozzle to print a seventy-pel-long row or set  236  ( FIG. 7 ) of side-by-side pels across the x-axis location of the tenth segment  222  from the left of the uppermost section  210 , for instance. After printing row  236 , the resistance of sensor  190  is monitored by sensor circuit  225 . Since the uppermost nozzle  228  has been tested “good”, the uppermost nozzle is assumed to have actually printed. If ASIC  62  reads a positive signal, this locates the uppermost nozzle at the y-direction coordinate of the tenth segment  222  from the left, and allows proper x-axis positioning for the rest of the nozzle fire row print passes. 
   If ASIC  62  does not read a positive signal, the uppermost nozzle print row is assumed to have printed to the right of sensor gap  196 . In this case, after a pause for drying, printer  226  uses the uppermost nozzle to print a row  238  of seventy dots or pels across the x-axis location of the ninth segment  222  from the left of the uppermost section  210 . ASIC  62  checks the resistance of sensor  190 . If there is still no change in resistance, incrementally leftward rows  240 ,  242  and  244  are sequentially printed, with ASIC  62  checking the resistance of sensor  190  and allowing time for drying between the printing of each row. After row  244  is printed, ASIC  62  senses a change in resistance of sensor  190 , and the starting segment  222 , i.e., the sixth segment  222  from the left, is thus located and associated with the uppermost nozzle  228 . 
   Printer  226  then uses the second uppermost nozzle to print a single-pel-tall row  246  ( FIG. 8 ) of dots across the seventh segment  222  from the left. After printing row  246 , the resistance of sensor  190  is monitored by sensor circuit  225 . If the second uppermost nozzle actually prints, ASIC  62  reads a positive signal and logs the nozzle as “good” in NVRAM  234 . 
   A single-pel-tall row of seventy pels is printed by all 300 nozzles. After each row is printed, the expected change in resistance of sensor  190  is verified, and the nozzle is logged as being “good” in NVRAM  234 . After a row is printed in the last segment  222 , i.e., the fortieth or rightmost, of a section  210 , the known x-position dislocation is shifted back to the first segment  222 , i.e., the first or leftmost, in the next section  210 . 
   When the above process has been completed, a processor, such as ASIC  62 , may then process print jobs and adjust printing to account for nozzles which were logged to NVRAM  234  as “bad” or “non-jetting”. 
   Cabling and connectors of the sensor of the present invention are simplified and cost-reduced because the sensor has only two terminals. The sensor base can be made many-up with standard flex-cable manufacturing methods, then processed through a laser cut process to make the one-pel gap. 
   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.