Patent Publication Number: US-6655775-B1

Title: Method and apparatus for drop weight encoding

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to inkjet printers. More particularly, the present invention relates to a technique for encoding drop weight for a particular printhead using resistance values thereby allowing the printer to compensate for manufacturing tolerances of the printhead. 
     Thermal inkjet printers operate by rapidly heating a small volume of ink and causing the ink to vaporize, thereby ejecting a droplet of ink through an orifice to strike a recording medium, such as a sheet of paper. When a number of orifices are arranged in a pattern, the properly sequenced ejection of ink from an orifice causes characters or other images to be printed upon the recording media as the printhead is moved relative to the recording medium. 
     The printhead typically includes an orifice plate having very small nozzles through which the ink droplets are ejected. Adjacent to the nozzles inside the printhead are ink chambers, where ink is stored prior to ejection. Ink is delivered to the ink chambers through ink channels that are in fluid communication with an ink supply. The ink supply may be contained in a reservoir proximate the printhead or in the case of “off-axis” printers, the ink supply may be spaced from the printhead. 
     Ejection of an ink droplet through a nozzle may be accomplished by quickly heating a volume of ink within the ink chamber. Rapid expansion of ink vapor forces ink within the chamber through the corresponding nozzle forming a droplet. This process is called “firing”. The ink in the chamber is heated with a heat transducer that is aligned with the corresponding nozzle. Typically, the heat transducer is a resistor, or piezoelectric transducer, but may comprise any substance or device capable of quickly heating the ink. 
     The inkjet printhead is often mounted in a print cartridge which contains some form of ink reservoir portion. In the manufacture of inkjet print cartridges and more specifically, inkjet printheads, manufacturing tolerances tend to result in variation in drop volume from one printhead to next. This drop volume variation results from manufacturing tolerances in orifice diameter, the heating element formation such as resistor size in the case of a resistive heating element, the ink chamber size, and the ink channel dimensions, to name a few. These manufacturing tolerances all tend to produce variations in ink drop volume from one printhead to the next. 
     Some printers use techniques such as drop counting to determine an amount of ink remaining. As a result of drop volume variation, it is difficult to determine the amount of ink remaining in the ink cartridge or external ink supply. Therefore, manufacturing tolerances resulting in drop volume variation make drop counting techniques less reliable. 
     In addition, this drop volume variation effects the output image quality formed on print media. The drops that are ejected onto the print media form small dots on the print media. In the case of text printing the drop volume variation tends to result in dot size variation resulting in poor dot overlap. Poor dot overlap in text images results in poor print quality. In the case of images which are formed having a varying intensities sometimes referred to as “grayscale images” the color intensity or hue is related to the dot density. For example, in color printing frequently cyan, magenta and yellow drops of ink are used to produce a gamut of colors. Drop weight variation among different colors alters the dot size and therefore alters dot coverage which significantly affects the color reproduction. For example, if the magenta drop volume is significantly higher than intended, a hue shift will result in the output image which seriously reduces the printed image quality. 
     One solution is to make use of manufacturing techniques which produce tighter manufacturing tolerances. One problem associated with this technique is that these manufacturing methods which provide improved tolerances tend to be costly which tend to increase the cost of the inkjet print cartridge. 
     SUMMARY OF THE INVENTION 
     The present invention is an inkjet print cartridge for use in an inkjet printing apparatus for forming images on print media. The inkjet print cartridge includes an inkjet printhead that is responsive to print control signals for ejecting ink drops onto print media. The inkjet printhead has a manufacturing tolerance associated therewith producing a range of drop weights. The inkjet printhead has a corresponding drop weight from the range of drop weights. Included with the inkjet print cartridge is an information storage device that is associated with the inkjet printhead for storing information for identifying a corresponding drop weight from the range of drop weights. 
     In the preferred embodiment the inkjet print cartridge the storage device is a circuit having a resistance value corresponding to the corresponding drop weight for the printhead. In the preferred embodiment, the inkjet printing apparatus receives the inkjet print cartridge. The inkjet printing apparatus includes an information reading device for reading the drop volume information associated with the inkjet printhead. The inkjet printing apparatus makes use of the drop volume information for compensating for the manufacturing tolerance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 depicts a drop weight distribution curve for printheads having a nominal drop weight of 6 nanograms. 
     FIG. 2 depicts a print cartridge of the present invention which includes an apparatus for encoding drop weight for the particular printhead. 
     FIG. 3 depicts the preferred embodiment of the present invention for encoding drop weight using a resistive network. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 depicts a normal or Gaussian distribution curve  9  for an inkjet printhead that is produced in a manufacturing environment. The normal distribution curve  9  tends to be representative of inkjet printheads which are formed in high volume using numerous manufacturing steps with each step having a manufacturing tolerance associated therewith. The distribution curve  9  of FIG. 1 represents a manufacturing process for forming inkjet printheads having a nominal drop weight of 6 nanograms. The distribution function for drop weight variation can be represented by the function shown in equation 1 as follows: 
     
       
           F ( x )=1/σ{square root over (2)}π(exp(−( x-x   0 ) 2  /2σ 2 ))  Equation 1: 
       
     
     where x 0  is the mean drop weight, x is the drop weight and F(x) is the distribution as a function of drop weight. The distribution curve  9  has an x-axis representing drop weight in nanograms and a y-axis representing the distribution function for printheads having a mean (cmean) equal to 6 nanograms and a standard deviation (csigma) equal to 1 nanogram. Therefore, the y-axis is representative of a percentage of printheads having a drop weight shown on the x-axis. This example is used merely to illustrate drop weight variation in printheads formed using similar manufacturing techniques. It is assumed in this example that the printhead manufacturing tolerances can be represented by the normal distribution shown in FIG.  1 . In addition, the normal distribution for a 6 nanogram printhead is shown for illustrative purposes, printheads may have different nominal drop weights or different standard deviations. Furthermore, the actual distribution curve may be differ from the normal distribution curve  9  depending on the particular manufacturing methods used. 
     The drop volume and drop weight are related. Because it tends to be easier to measure drop weight than drop volume the method and apparatus of the present invention utilizes drop weight information. However, the method and apparatus is equally applicable to drop volume information as well. 
     The area under this curve  9  represents the number of printheads having a given drop weight range. Therefore, using this distribution 68.3 percent of the printheads are within 1 sigma or 1 nanogram of the nominal, 6 nanograms, 95.6 percent are within the 2 sigma range and 99.7 percent are within the 3 sigma range. 
     Some of the printheads will have drop weights of +/−3 sigma which corresponds to drop weights of 3 nanograms and 9 nanograms. These 3 and 9 nanogram printheads have a drop weight variation that is 50% from the mean of 6 nanograms. Therefore, if printer parameters are chosen for the nominal drop weight of 6 nanograms for instance, then some printheads will be used which will have drop weights of 3 and 9 nanograms toward the outer edge of the manufacturing range. It is likely that this manufacturing tolerance will result in performance problems such as dot overlap problems on the print medium. The variation in print overlap due to drop weight variation tends to reduce the quality of the output image. In addition, printers which use drop counting techniques for monitoring ink consumption may be off by as much as 50% due to this drop weight variation of the printhead. 
     If the printheads are sorted and only the printheads having a drop weight variation of one sigma from the nominal are used then this would be 68.3 percent of the printheads. The remaining 31.7 percent of the printheads would be unusable resulting in waste as well as increased manufacturing costs. 
     FIG. 2 depicts a preferred embodiment of the inkjet print cartridge  10  of the present invention for use in the inkjet printer  12  for forming images on print medium. The inkjet printer  12  including a cartridge mount  13  for receiving one or more inkjet print cartridges  10 . The inkjet print cartridge  10  includes an inkjet printhead  14  that is responsive to print control signals for ejecting ink drops onto print media. The inkjet printhead  14  has a manufacturing tolerance associated therewith producing a range of drop weights. The inkjet printhead  14  has a corresponding drop weight from the range of drop weights. The inkjet print cartridge  10  includes an information storage device  16  that is associated with the inkjet printhead  14  for storing information for identifying the corresponding drop weight. 
     In the preferred embodiment, the inkjet print cartridge  10  includes a pen body which defines a reservoir  18 . The reservoir  18  is configured to hold a quantity of ink. The printhead  14  is fit to the bottom  20  of the print cartridge  10  and is controlled by electrical interconnects  21  for ejecting ink droplets from the printhead  14 . The printhead  14  defines a set of nozzles  22  for expelling ink, in a controlled pattern, during printing. Each nozzle  22  is in fluid communication with a firing chamber (not shown) that is defined within the printhead  14 . 
     In one preferred embodiment, the print cartridge  10  includes an ink supply within the cartridge reservoir  18 . Alternatively, the ink cartridge  10  may be configured for use with (off-axis) ink supplies which are spaced from the print cartridge  10  and in fluid communication with the print cartridge  10 . Regardless of where the ink supply is located, a supply conduit (not shown) conducts ink from the ink reservoir  18  to one or more ink channels (not shown) defined within the print cartridge  10 . The ink channels are configured so that ink moving therethrough is in fluid communication with each of the firing chambers and hence each nozzle  22 . 
     The information storage device  16  in the preferred embodiment is a circuit connected between a pair of terminals  24  and  26 . In the preferred embodiment the circuit provides a resistance between the terminals  24  and  26  which is indicative of the drop weight of the particular printhead  14 . In this preferred embodiment a series of switches  28  are provided for selecting a resistance value for the circuit between terminals  24  and  26  for identifying the drop weight of the printhead  14 . 
     FIG. 3 depicts the preferred embodiment of the storage device  16  for identifying the drop weight of the printhead  14 . The storage device  16  includes a plurality of resistors  30  connected in parallel between terminals  24  and  26 . Connected in series with each of the resistors  30  are switches  28 . The resistance between terminals  24  and  26  are selected by selectively activating switches  28 . Once the drop weight of the printhead  14  is determined, the appropriate switches  28  are activated to select a resistance value corresponding to the drop weight of the printhead  14 . 
     An information retrieval device  32  having a pair of terminals  34  and  36  are configured for engaging the corresponding terminals  24  and  26 , respectively of the storage device  16  for retrieving the drop weight of the printhead  14  from the storage device  16 . In the preferred embodiment, the information retrieval device  32  is a resistance sense circuit that is located on the printer  12 . The terminals  34  and  36  are positioned such that when the ink cartridge  10  is properly installed in the printer  12  the terminals  34  and  36  of the information retrieval device  32  are electrically connected to the terminals  24  and  26  of the storage device  16  so that the drop weight information stored in the storage device  16  can be retrieved by the information retrieval device  32  so that the printer  12  can properly compensate for any drop weight variation by the printhead  14 . 
     The storage device  16 , in the preferred embodiment, makes use of resistors  30  which have either the same or nearly the same resistance value. For this preferred embodiment, assuming the total value of the resistance for the storage device  16  circuit is equal to R 1  where the circuit has n resistors with each resistor has a resistance value of R. For this preferred embodiment the above relationship can be represented by equation 2. 
     
       
         1 /R   t   =n/R   Equation 2: 
       
     
     For the case where one of the resistors R is not connected because the switch  28  is not activated then the resistance between connectors  24  and  26  would be related by equation 3. 
     
       
         1 /R   t =( n −1)/ R   Equation 3: 
       
     
     Using this technique a resistance value is preassigned for each group of drop weights of interest for the printhead  14 . Once the printhead  14  or print cartridge  10  is inserted into the printer  12 , the resistance is measured by the information retrieval device  32  of the printer  12  for determining ink usage as well as ink coverage for improving the quality and reliability of the printer  12 . 
     In the preferred embodiment the storage device  16  is formed by a conductive layer such as copper on an insulating layer such a polymer material such as polyimid. The conductive portions are preferably defined using a photolithographic technique and an etching technique. The switches  28  are formed by defining a gap or spacing in the copper conductive traces thereby setting each of the switches  28  to an inactive mode or nonconductive mode. Once the drop weight is determined for the printhead  14 , the switches  28  are selectively activated by selectively placing an electrically conductive material between the gaps or spacing in the conductive traces thereby electrically connecting the selected resistor between the pair of terminals  24  and  26 . In the preferred embodiment the conductive material is a conductive epoxy is placed between the gaps or spacing for electrically connect the copper traces thereby activating the switch  28 . As more resistors  30  are connected in parallel between the pair of terminals  24  and  26  the resistance between the pair of terminals is altered. The number of switches  28  which are activated is related to the drop weight of the printhead  14 . 
     The drop weight of the printhead  14  is determined either directly or indirectly. The direct method for determining the drop weight of the printhead  14  is to fire or eject a known number of drops into a collection pan in a weighing scale. The weight is recorded and the average drop weight can then be determined. The indirect method for determining the drop weight for printhead  14  is by printing a pattern of dots on a medium. The drop weight can then be inferred by spot size. Spot size may be measured using machine vision in the preferred embodiment. The drop weight is then calculated from the spot size based on experimental correlation which is stored in a computer. Based on the data of drop weight, printheads can then be sorted according to ranges of drop weight. For example, the 3-9 nanogram drop weight range as disclosed in FIG. 1 may be subdivided into 3 groups each group consisting of a 1.5 nanogram range. A code is then used to activate or program switches  28  such that when the cartridge  10  is inserted into the printer  12  the printer  12  properly compensates for the drop weight of the particular printhead  14 . 
     Alternatively, printhead parameters such as resistor, orifice, chamber dimensions etc. can be related by a statistical model correlation equation to drop weight based on experimental measurements of drop weight and printhead parameters. For a given printhead knowing the critical dimensions, a drop weight can be calculated based on model equation and the pen can be encoded with this drop weight using the apparatus of the present invention. 
     The storage device  16  has been described as a resistor array which has a resistance value that is selectable or programmable. Alternatively, the storage device  16  can be a variety of devices for storing information indicative of drop weight for the printhead. For example, the storage device  16  can be a plurality of capacitive elements that are configured to provide a known capacitive value representative of drop weight. The information retrieval device  32  is capable of determining the drop weight based on the capacitance value. Alternatively, the storage device  16  can be a label having an indicia indicative of drop weight. The label is affixed to the print cartridge  10  once the drop weight is determined. The information retrieval device  32  within printer  12  is a label reading device for determining the printhead drop weight. In another alternative embodiment the storage device  16  is some form of electronic memory such as a read only memory (ROM), read access memory (RAM) or some form of programmable device such as electrically erasable read only memory (EEPROM) for storing drop weight information. The information retrieval device  32  within printer  12  for these examples is a suitable device for reading drop weight information from these devices. 
     The present invention provides a low cost technique for identifying or tagging printheads by drop weight. In the case of color printers drop weights for each of the colors can be encoded or identified by the printhead. The printer which these printheads are installed are capable of reading these tags or drop weight information, thus allowing the printer to compensate for drop weight variation from printhead to printhead. By providing this drop weight information to the printer the printer is capable of forming high quality output images using printheads having a wide range of drop weights. Because printheads having a wider range of drop weights can be used the manufacturing costs of the printhead is reduced.