Abstract:
An ink detection 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  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to ink jet printers, and, more particularly, to an ink sensor for an ink jet printer.  
           [0003]    2. Description of the Related Art  
           [0004]    Occasionally, under error conditions such as a paper jam or the like, an ink jet printer may print on the platen. Another cause of printing on the platen is using paper that is too narrow for the print job. This is a very detrimental situation, because the ink on the platen becomes very gummy, and contributes to future paper jams, ink smear on the back side of subsequent pages, degraded roller performance, head-to-paper gap problems, etc.  
           [0005]    The presence of paper is generally sensed when it enters the nip of the pinch roller just before the paper enters the print zone. Sometimes, paper will jam at this point, such that the printer senses paper present, but no paper is actually in the print zone. Presently, ink jet printers do not have a means to detect that ink is then printing on the platen.  
           [0006]    What is needed in the art is a simple, low-cost device for sensing the jetting of ink droplets within an ink jet printer.  
           [0007]    Present-day ink jet printers with disposable printheads often continue to function even when the printhead cartridge or bottle has been depleted of ink. This is because they do not have a means to accurately monitor ink usage and accurately detect when ink is depleted. This requires that a user rerun a job if the ink has run dry during his job. For a long job this a severe nuisance.  
           [0008]    Many schemes exist in the prior art to sense ink out. These schemes meet with varying degrees of functionality and accuracy. One example is a dot-counting scheme wherein ink usage and an “ink-out” point is estimated based on the dot count. Another scheme uses an optical sensor and a clear ink bottle to “view” the ink level in a special cavity of the bottle. Another method relies on the back pressure of a collapsed lung internal to the bottle. There are also capacitive, resistive, carrier mass measurement and many other types of sensing ink level. Among the disadvantages of the prior art sensors are that they are expensive, they are inaccurate, or they disable the printhead with an amount of ink still in the head.  
           [0009]    What is needed in the art is a simple, low-cost, accurate device for sensing when an inkjet printhead has run dry.  
           [0010]    There is an optimal operating point for ink jet printheads such that when they are operated at too low a supply voltage they do not jet, but when they are operated at too high a supply voltage they will suffer reduced life. Optimally, it is desired to operate the printhead at voltage just above that required to begin jetting the nozzles, i.e., the point of nucleation. Ideally, it is desired to automatically detect this point for an individual printer, as this voltage varies from printer to printer due to component tolerances, environmental factors, etc. Thus autonucleation detection is a desired function for an inkjet printer.  
           [0011]    Schemes exist in the prior art to detect printhead nucleation voltage. One example is a printhead temperature sensing scheme wherein the voltage is stepped up repeatedly, and the printhead temperature is monitored. The temperature increases with the voltage until nucleation, after which there is a detectable fall in temperature for a given fire voltage because energy and therefore heat is being carried away from the head with the fired ink droplets. Thus, the nucleation voltage is inferred from the inflection point of a plot of temperature versus voltage.  
           [0012]    A second scheme involves setting a voltage, printing a pattern on a page, and sensing with an optical sensor whether the pattern actually printed. If not, the voltage is stepped up, the pattern is printed, optical sensing is performed, etc, and this process is repeated until the printed pattern is sensed, indicating that the nucleation voltage has been reached.  
           [0013]    Among the disadvantages of the prior art schemes are that they are generally expensive and complex, requiring optical or temperature sensing, and perhaps requiring printing of a test page, etc. Also, the determination of the inflection point of the temperature versus voltage plot is prone to inaccuracies, as the inflection is a subtle one and the system is prone to noise.  
           [0014]    What is needed in the art is a simple, low-cost, accurate device for detecting autonucleation.  
         SUMMARY OF THE INVENTION  
         [0015]    The present invention provides a simple, low-cost sensor apparatus that can detect whether ink droplets are being jetted within an ink jet printer, and whether the ink drops are being jetted in a certain area of the printer, such as on the platen.  
           [0016]    The present invention also provides a device for sensing when an inkjet printhead has run dry, and pausing a job at the page on which the head runs dry, and allowing the user to replace the head cartridge. The job is then resumed such that the driver reruns the last page and completes the job.  
           [0017]    The present invention further provides a low-cost, simple device that can detect when autonucleation has taken place in an ink jet printer.  
           [0018]    The invention comprises, in one form thereof, an ink detection sensor for an ink jet printer. Two terminals define 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.  
           [0019]    The invention comprises, in another form thereof, a method of operating an ink jet printer, including monitoring whether ink is impinging upon a platen of the ink jet printer. A print job is stopped if it is detected that ink is impinging upon the platen.  
           [0020]    An advantage of the present invention is that it can be determined whether ink is being jetted onto a platen of an ink jet printer.  
           [0021]    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.  
           [0022]    Yet another advantage is that no special alignment of the sensor in the printer is required. This allows ease of printer manufacturing assembly.  
           [0023]    A further advantage is that it can be determined whether an ink jet printer is out of ink.  
           [0024]    A still further advantage is that autonucleation of ink within an ink jet printer can be detected.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]    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:  
         [0026]    [0026]FIG. 1 is an overhead schematic view of one embodiment of a slotted sensor of the present invention;  
         [0027]    [0027]FIG. 2 is a schematic view of one embodiment of a sensing circuit in which the sensor of FIG. 1 can be incorporated;  
         [0028]    [0028]FIG. 3 is a front, sectional, perspective view of an ink jet printer including the sensing circuit of FIG. 2;  
         [0029]    [0029]FIG. 4 is an overhead schematic view of another embodiment of a slotted sensor of the present invention;  
         [0030]    [0030]FIG. 5 is an overhead view of yet another embodiment of a slotted sensor of the present invention;  
         [0031]    [0031]FIG. 6 is a schematic view of one embodiment of a sensing circuit in which the sensor of FIG. 1 can be incorporated;  
         [0032]    [0032]FIG. 7 is a sectional, perspective view of a sensor cleaning mechanism of the printer of FIG. 3; and  
         [0033]    [0033]FIG. 8 is a schematic diagram of one embodiment of a printhead voltage adjustment circuit of the present invention. 
     
    
       [0034]    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  
       [0035]    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 {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. 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., gap  48  may receive several print “sprays”.  
         [0036]    Slotted sensor  40  can be incorporated in a sensing circuit  120 , as shown in FIG. 2. 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, which is translated into a positive digital pulse that is sent to an input of a latch  122 . Latch  122  drives an interrupt to printer application specific integrated circuit (ASIC)  62  to indicate that the printed dot column has been printed in gap  48  of sensor  40 .  
         [0037]    One embodiment of the print-on-platen detection method of the present invention includes positioning at least one sensor  40  in the horizontal print path of carrier  30  (FIG. 3) within ink jet printer  64 . Any time ink is sprayed or jetted into gap  48  of one or more of sensors  40 , the output of comparator  60  flips and latch  122  is triggered. The latch output is connected to a processor interrupt of ASIC  62 . At the interrupt, the firmware stops the print job and indicates an error to the user.  
         [0038]    In another embodiment, a redundant sensor  72  (FIG. 4) 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.  
         [0039]    In yet another embodiment (FIG. 5), 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.  
         [0040]    Another embodiment of a sensing circuit  58  is shown in FIG. 6. A change in resistance in sensor  40  from several hundred megohms to just a few megohms is translated into a positive digital pulse that is sent to printer application specific integrated circuit (ASIC)  62  to indicate when a column of ink dots has been printed in gap  48  of sensor  40 .  
         [0041]    One embodiment of an ink-out detection method of the present invention includes positioning at least one sensor  40  in the horizontal print path of carrier  30  within ink jet printer  64 . An alignment procedure is performed to locate the placement of gap  48  in sensor  40 . Carrier  30  is positioned at the approximate position of gap  48 , and columns of ink dots are sequentially sprayed at sensor  40  such that each pass shifts the column of printed dots by one pel closer to gap  48  until sensor  40  detects the presence of the dot column.  
         [0042]    Dot counting or another form of rough gauging is performed until the ink is determined to be at the last 10% of its expected page count. Rough gauging is used for the majority of the cartridge life to minimize the need for cleaning sensor  40 . During the last 10% of the life of printhead  34 , after each printed page, printer  64  positions printhead  34  over gapped sensor  40 , sprays a column of dots, and determines the presence or absence of ink. When the ink spray is no longer sensed, the job is paused, and the user is alerted to the ink-out condition. The user may replace the cartridge, after which the page may be reprinted and the job completed.  
         [0043]    Periodically, sensor  40  is automatically wiped with a cleaning pad  124  (FIG. 7) in order to remove ink therefrom. A motor  126  pivots a flange  128  about axis  129  such that sensor  40  is wiped across pad  124 , and motor  126  then pivots sensor  40  back to its original position.  
         [0044]    The ink-out detection method described above can also be performed using sensor  72  or sensor  102 .  
         [0045]    One embodiment of an autonucleation detection method of the present invention includes positioning at least one sensor  40  in the horizontal print path of carrier  30  within ink jet printer  64 . An alignment procedure is performed to locate the placement of gap  48  in sensor  40 . Carrier  30  is positioned at the approximate position of gap  48 , and columns of ink dots are sequentially sprayed at sensor  40  such that each pass shifts the column of printed dots by one pel closer to gap  48  until sensor  40  detects the presence of the dot column.  
         [0046]    Printhead  34  is positioned over gap  48  in sensor  40 . The printhead voltage is set to a value below the range that would jet the nozzles. An attempt is then made to fire a column of ink dots from printhead  34  onto gap sensor  40 . The printhead voltage is then repetitively stepped up to the next incremental value through the range that would jet the nozzles. At each increment, printhead  34  is fired in an attempt to jet a column of ink dots onto gap sensor  40 .  
         [0047]    When the ink spray is sensed by sensor  40 , nucleation has been detected. The voltage may be incremented a few more steps to provide a reasonable safety margin. At this point, the printhead voltage is optimally set to be as low as possible to still fire reliably, which results in the longest possible printhead life.  
         [0048]    One embodiment of a circuit  130  for adjusting the printhead voltage under firmware/processor control is shown in FIG. 8. Circuit  130  includes a digital resistance selection bus  132 , a digital potentiometer  134  and a printhead voltage regulator  136 . Regulator  136  has an output  138  that is nominally 12 volts, but which can be varied between 10 volts and 14 volts.  
         [0049]    The autonucleation detection method described above can also be performed using sensor  72  or sensor  102 .  
         [0050]    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.  
         [0051]    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.