Abstract:
A thermal ink jet printing apparatus maintains stable printing output as certain characteristics of the apparatus change over its operational lifetime. The apparatus includes an ink jet print head with resistive heating elements for receiving electrical energy pulses having a voltage level and for transferring heat energy pulses having a desired energy level into adjacent ink based on the electrical energy pulses. The print head includes nozzles associated with the resistive heating elements through which droplets of the ink are ejected when the heat energy pulses are transferred into the ink. The apparatus further includes a printer controller in electrical communication with the print head. The printer controller determines a pulse count indicative of a number of electrical energy pulses, applies the electrical energy pulses having a first pulse width to the resistive heating elements when the pulse count is less than a threshold value, and applies the electrical energy pulses having an adjusted pulse width to the resistive heating elements when the pulse count exceeds the threshold value. The difference in the first and the adjusted pulse widths compensates for changes in the electrical resistance of the resistive heating elements over time.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention is generally directed to thermal ink jet printing. More particularly, the invention is directed to a method and apparatus for maintaining desired levels of heat energy transferred into ink to form ink droplets as characteristics of an ink jet print head change over its operational lifetime.  
         BACKGROUND OF THE INVENTION  
         [0002]    Generally, thermal ink jet print head chips consist of several thin film layers, including a resistor layer, conductor layer, dielectric layer, and protection layer. When electrical current is passed through a resistive heating element formed in the resistor layer, ink adjacent to the heating element is superheated and forms a bubble that causes an ink droplet to be expelled from an adjacent nozzle.  
           [0003]    Many thermal ink jet print heads incorporate a tantalum aluminum (TaAl) thin film as the resistor layer in which the resistive heating elements are formed. Over time, a TaAl thin film experiences material degradation due to current and temperature stressing as electrical current pulses are applied to the heating elements. The material degradation mechanisms include aluminum segregation from the TaAl film, recrystallization of the TaAl under high temperatures, and electromigration of aluminum from the TaAl film. This degradation causes a gradual decrease in the electrical resistance of the heating elements over time.  
           [0004]    Many current ink jet printers apply one voltage level (rail voltage) to the resistive heating elements to pass electrical current through the elements, and this voltage level is not changed over the lifetime of a print head. With a constant rail voltage, any decrease in heating element resistance, such as by material degradation, causes a corresponding increase in the current flowing through the heating elements. An increase in current causes a corresponding increase in the heat energy generated by the heating elements, and an increase in the temperature at the surface of the heating elements. If surface temperatures rise too high, extensive ink kogation may occur at the surface of the heating elements. Also, increased current levels cause even greater electromigration or segregation of the aluminum in the TaAl film, which is further detrimental to heater reliability.  
           [0005]    Therefore, a system is needed for maintaining stable heat energy levels at the surfaces of the resistive heating elements over the operational lifetime of an ink jet print head.  
         SUMMARY OF THE INVENTION  
         [0006]    The foregoing and other needs are met by a method of operating a thermal ink jet print head having nozzles through which ink is ejected when energy pulses having a desired pulse energy are applied to resistive heating elements associated with the nozzles. Each of the resistive heating elements has a heater resistance which tends to change over the operational lifetime of the print head. The method provides stable ink ejecting characteristics over the lifetime of the print head by compensating for the change in heater resistance. The method includes applying energy pulses having a first pulse width to the resistive heating elements, and counting the energy pulses to determine a pulse count. When the pulse count exceeds a threshold value, pulses having an adjusted pulse width are applied to the resistive heating elements, where the adjusted pulse width accounts for the changes in the heater resistance during the operational lifetime of the print head.  
           [0007]    Preferred embodiments of the method include accessing a total print head resistance value which is based at least in part upon the heater resistance and resistances of circuit components in series with the resistive heating elements, accessing a heater resistance value related to the heater resistance, accessing a print head voltage value, accessing a first pulse energy value related to the desired pulse energy, and determining the first pulse width based upon the heater resistance value, the total print head resistance value, the print head voltage value, and the first pulse energy value. Preferred embodiments further include accessing a second pulse energy value related to the desired pulse energy and determining the adjusted pulse width based upon the heater resistance value, the total print head resistance value, the print head voltage value, and the second pulse energy value.  
           [0008]    In another aspect, the invention provides a thermal ink jet printing apparatus for maintaining stable printing characteristics. The apparatus includes an ink jet print head having resistive heating elements for receiving electrical energy pulses having a voltage level and for transferring heat energy pulses having a desired energy level into adjacent ink based on the electrical energy pulses. The print head includes nozzles associated with the resistive heating elements through which droplets of the ink are ejected when the heat energy pulses are transferred into the ink. The apparatus further includes a printer controller in electrical communication with the print head. The printer controller determines a pulse count indicative of a number of electrical energy pulses, applies the electrical energy pulses having a first pulse width to the resistive heating elements when the pulse count is less than a threshold value, and applies the electrical energy pulses having an adjusted pulse width to the resistive heating elements when the pulse count exceeds the threshold value. The differences in the first and the adjusted pulse widths compensate for changes in the electrical resistance of the resistive heating elements over the operational lifetime of the print head. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    Further advantages of the invention will become apparent by reference to the detailed description of preferred embodiments when considered in conjunction with the drawings, which are not to scale, wherein like reference characters designate like or similar elements throughout the several drawings as follows:  
         [0010]    [0010]FIG. 1 depicts a thermal ink jet print head according to a preferred embodiment of the invention;  
         [0011]    [0011]FIG. 2 is a functional block diagram of a thermal ink jet print head connected to a printer controller according to a preferred embodiment of the invention;  
         [0012]    [0012]FIG. 3 depicts the application of a rail voltage to print head resistances according to a preferred embodiment of the invention;  
         [0013]    [0013]FIGS. 4A and 4B depict a functional flow diagram of a preferred method for adjusting the pulse width of ink-firing pulses in an ink jet print head; and  
         [0014]    [0014]FIG. 5 depicts a functional flow diagram of an alternative method for adjusting the pulse width of ink-firing pulses in an ink jet print head.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]    [0015]FIG. 1 depicts an ink jet print head  10 , such as may be used in a thermal ink jet printer. The print head  10  includes an integrated circuit chip, also referred to herein as an ink jet heater chip  12  which, as described in more detail below, contains resistive heating elements, driver circuits, logic devices, and memory devices. An array of nozzles  14  are provided on the print head  10  through which droplets of ink are selectively ejected when corresponding heating elements in the heater chip  12  are activated. On the print head  10  are a set of electrical contacts  18  which make connection with a corresponding set of contacts in the printer when the print head  10  is installed in the printer. Electrical traces provided in the dashed-outline region  16  connect the contacts  18  to the heater chip  12 .  
         [0016]    Shown in FIG. 2 is a functional block diagram of the print head  10  connected to a printer  20 . Within the printer  20  is a microprocessor controller  22  that provides print control signals to the print head  10  based on print data from a host computer. The print control signals include a print head voltage signal, also referred to herein as a rail voltage, on the line  24 , and an encoded nozzle selection or address signal on the line  26 . Preferably, the rail voltage on the line  24  is provided as a pulsed signal, having a voltage amplitude in the 7-11 volt range, and having a pulse width in the 0.5 to 3.0 μs range. As described in more detail hereinafter, the invention sets the pulse width of the rail voltage pulses to provide an optimum energy density on the surface of the heating elements of the print head  10 .  
         [0017]    As depicted in FIG. 2, the line  24  provides the rail voltage to a driver  28 , such as a MOSFET device, which acts as a switch. The on/off state of the driver  28  is determined, at least in part, upon a selection signal from a selection logic circuit  29 . If the driver  28  is “on”, a current I i  flows through a heating element  30  and through the driver  28  which is in series with the heating element  30 . The heating element  30  of the preferred embodiment is constructed from a tantalum aluminum (TaAl) thin film, and has an electrical resistance referred to herein as R H . Due to the resistance R H , the current Ii flowing through the heating element  30  generates heat energy on the surface of the heating element  30 . This heat energy is transferred into ink adjacent the heating element  30 , thereby causing the ink to nucleate and force a droplet of ink outward through an associated one of the nozzles in the nozzle array  14 .  
         [0018]    The number of drivers and heating elements on a heater chip of a print head is typically in the hundreds. However, to avoid unduly complicating FIG. 2, only one driver  28  and one heating element  30  are depicted. One skilled in the art will appreciate that the present invention is applicable to a print head having any number of heating elements.  
         [0019]    The driver  28 , the line  24 , and the contacts  18  introduce resistance in series with the heating element  30 . This series resistance, as depicted in FIG. 3, is referred to herein as R S . The sum of R S  and R H  is referred to herein as the total resistance R T . The current I i  flowing through the heating element  30  is expressed as:  
                 I   i     =     V     R   T         ,     w                 h                 e                 r                 e                 V                 i                 s                 t                 h                 e                 r                 a                 i                 l                 v                 o                 l                 t                 a                 g                   e   .               (   1   )                               
 
         [0020]    where V is the rail voltage. (1)  
         [0021]    The heat energy at the surface of the heating element  30  produced by a pulse of the current I i  may be expressed as:  
           E   P   =T   P   ×I   1   2   ×R   H ,  (2)  
         [0022]    where E P  is the heat energy produced by the current pulse and T P  is the pulse width. This relationship may also be expressed as:  
               E   P     =         T   P     ×       (     V     R   T       )     2     ×     R   H       =       T   P     ×       (     V       R   H     +     R   S         )     2     ×       R   H     .                 (   3   )                               
 
         [0023]    As equation (3) indicates, if the resistance R H  were to decrease over time, such as due to material degradation of the TaAl thin film, the pulse heat energy E P  would increase. During design of the print head  10 , the resistance R H , the voltage V, and the pulse width T P  are set to provide an optimum energy density on the surface of the heating element  30 . This optimum energy density is preferably high enough to cause nucleation of the ink to form an ink droplet moving at a desired velocity, but not so high as to cause kogation, or scalding, of the ink at the surface of the heating element  30 . Significant kogation impedes heat transfer and causes degradation in print quality. Thus, a significant decrease in the resistance R H  leads to degradation in print quality if no compensation is provided to reduce the energy density at the surface of the heating element  30 . As discussed in more detail hereinafter, the present invention provides this needed compensation by adjusting the pulse width T P  to account for changes in the resistance R H  over time.  
         [0024]    As shown in FIG. 2, the print head  10  includes a nonvolatile memory device  32 , such as an EEPROM device, for storing values related to the pulse width T P . In the preferred embodiment of the invention, the memory device  32  stores a value for the rail voltage V, a value for the initial heater resistance R H , a value for the total resistance R T , a value for a pulse count, a value for a pulse count threshold, and values related to an initial pulse energy E 1  and an adjusted pulse energy E 2 . As described below, the controller  22  accesses the memory device  32  to retrieve one or more of these values, and calculates an optimum pulse width based thereon.  
         [0025]    Depicted in FIGS. 4A and 4B is a process for implementing a one-time adjustment in the pulse width T P  to compensate for changes in the resistance R H  over the operational lifetime of the ink jet print head  10 . The process is preferably begun during the manufacture of the ink jet print head  10  by recording in the memory device  32  the values related to print head characteristics which will be used in determining an optimum pulse width for the ink-firing pulses (step  100 ). In the preferred embodiment, these values include the rail voltage V, the initial heater resistance R H , and the total resistance R T , each of which is preferably measured during testing stages of the print head assembly process. Predetermined values related to the initial pulse energy E 1  and the adjusted pulse energy E 2  are also stored in the memory device  32 . The initial pulse energy value E 1  represents the desired value of heat energy generated by the heating element  30 . The adjusted pulse energy value E 2  represents a change in energy to account for the expected change in heating element resistance R H  after a predetermined number of firing pulses.  
         [0026]    In the preferred embodiment, the process for adjusting the pulse width is carried out when the printer  20  is powered on, when a print head maintenance routine is performed, or when a new print head  10  is installed in the printer  20 . If any one of these events occurs (step  102 ), the printer controller  22  accesses the rail voltage value V and the total resistance value R T  from the print head memory device  32  (step  104 ), and calculates the initial current value I i , preferably based on equation (1) (step  106 ).  
         [0027]    During the operational lifetime of the print head  10 , a running count is kept of the number of ink-firing pulses generated by the print head  10 . Preferably, since this pulse count value is associated with a particular print head  10 , it is stored in the print head memory device  32 . Alternatively, the pulse count value may be stored in memory in the printer  20 . The controller  22  accesses the pulse count value and determines based thereon how many ink-firing pulses have been generated by the installed print head  10  (step  108 ). The subsequent steps in the process are determined by whether the pulse count exceeds a predetermined threshold value.  
         [0028]    Experiments conducted on a particular print head manufactured by the assignee of this invention have indicated that about 50% of the reduction in the heating element resistance R H  due to thin film material degradation occurs prior to the pulse count reaching about 7.5 million. Thus, in the most preferred embodiment of the invention, the threshold value is about 7.5 million. However, it should be appreciated that the rate of change in heating element resistance R H  may vary from one print head design to the next, such that different threshold values may be selected based upon characteristics that vary from one print head design to the next. Thus, it should be appreciated that the invention is not limited to any particular threshold value.  
         [0029]    As depicted in FIGS. 4A and 4B, if the controller  22  determines that the pulse count value is less than the threshold value (step  110 ), the controller  22  accesses the heating element resistance value R H  and the initial pulse energy value E 1  from the print head memory device  32  (step  112 ). In the preferred embodiment, the controller  22  then calculates an initial or first pulse width value T 1  according to:  
               T   1     =         E   1         I   i   2     ×     R   H                           (     s                 t                 e                 p                 114     )     .               (   4   )                               
 
         [0030]    The controller  22  then sets the pulse width of the ink-firing pulses on the line  26  according to the value T 1  (step  116 ). The pulse width T 1  is preferably maintained in generating ink-firing pulses (step  118 ) for all subsequent printing operations which take place prior to the next occurrence of any one of the conditions of step  102 .  
         [0031]    If the controller  22  determines at step  110  that the pulse count value is greater than the threshold value, the controller  22  accesses the heating element resistance value R H  and the adjusted pulse energy value E 2  from the print head memory device  32  (step  120 ). In the preferred embodiment, the controller  22  then calculates an adjusted or second pulse width value T 2  according to:  
               T   2     =         E   2         I   i   2     ×     R   H                           (     s                 t                 e                 p                 122     )     .               (   5   )                               
 
         [0032]    The controller  22  then sets the pulse width of the ink-firing pulses on the line  26  according to the value T 2  (step  124 ). In this embodiment of the invention, the adjusted pulse width T 2  is preferably maintained in generating ink-firing pulses (step  118 ) for all subsequent printing operations during the lifetime of the print head  10 .  
         [0033]    As described above, the preferred embodiment of the invention stores several values in the memory  32  related to the initial measured resistances and rail voltage, the calculated initial current, the pulse count, the pulse count threshold value, and the initial and adjusted energy levels, and uses these stored values to calculate initial and adjusted pulse widths. In an alternative embodiment of the invention, only pulse width values are stored, such as an initial pulse width value to be used when the pulse count is less than a threshold value, and an adjusted pulse width value to be used when the pulse count is greater than a threshold value. For example, the initial pulse width value T 1  may be determined during the manufacture of the print head according to:  
                 T   1     =         E   1     ×       (       R   S     +     R   H       )     2           V   2     ×     R   H           ,           (   6   )                               
 
         [0034]    where V, R S , and R H  are measured values as described above, and E 1  is the desired pulse energy to be maintained throughout the lifetime of the print head  10 . Similarly, the adjusted pulse width T 2  is determined and stored during the manufacture of the print head according to:  
                 T   2     =         E   1     ×       (       R   S     +     R   2       )     2           V   2     ×     R   2           ,           (   7   )                               
 
         [0035]    where R 2  is the predicted heating element resistance value after the pulse count exceeds the threshold value.  
         [0036]    In one embodiment of the invention, multiple pulse width adjustments are made during the lifetime of the print head  10  to compensate for changes in the heating element resistance R H . In this embodiment, N number of count threshold values are stored in memory, either in the print head memory  32  or in memory associated with the printer controller  22 . As described in more detail below, the pulse width of the ink firing pulses is adjusted in a number of steps as the pulse count exceeds a corresponding number of count threshold values.  
         [0037]    As with the previously-described embodiments, the process of this embodiment is preferably begun during the manufacture of the ink jet print head  10  by recording in the memory device  32  values related to print head characteristics that are used in determining an optimum pulse width for the ink-firing pulses (step  200 ). These values preferably include the rail voltage V, the initial heater resistance R H(1) , the series resistance R S , and the desired pulse energy value E 1 . The printer controller  22  accesses these stored values (step  202 ) and calculates an initial pulse width T N  (for adjustment step N=1) based on the following expression:  
               T   N     =           E   1     ×       (       R   S     +     R     H        (   N   )           )     2           V   2     ×     R     H        (   N   )                               (     s                 t                 e                 p                 204     )     .               (   8   )                               
 
         [0038]    The controller  22  accesses the pulse count value from the print head memory device  32  or from memory associated with the controller  22 , and determines based thereon how many ink-firing pulses have been generated by the print head  10  up to that point in the print head lifetime (step  206 ). The controller  22  accesses the pulse count threshold, also referred to as THRSHLD N , (where N=1) and determines whether the count value exceeds THRSHLD N . If not, the initial pulse width is maintained in generating the ink-firing pulses (step  210 ).  
         [0039]    If the pulse count exceeds THRSHLD N , then N is incremented by one (step  212 ), and a new heating element resistance value R H(N)  is calculated. Preferably, the new resistance value is calculated (step  214 ) according to:  
           R   H(N)   =R   H(1)   −ΔR   H ,  (9)  
         [0040]    where ΔR H  is a resistance change value calculated according to:  
         Δ R   H   =R   H(1)   ×[A+B ×log( PC )].  (10)  
         [0041]    In equation (10), A and B are experimentally-determined constants, and PC is the current pulse count.  
         [0042]    Based on the new resistance value R H(N) , the controller  22  calculates an adjusted pulse width value T N*  according to:  
                 T     N   *       =         T     N   -   1       2     +           E   1     ×       (       R   S     +     R     H        (   N   )           )     2         2   ×     V   2     ×     R     H        (   N   )                             (     s                 t                 e                 p                 216     )           ,           (   11   )                               
 
         [0043]    and sets the pulse width accordingly (step  218 ). The newly-adjusted pulse width value T N*  is used in generating the ink-firing pulses while the pulse count value is between the pulse count thresholds THRSHLD N  and THRSHLD N-1 . For this embodiment, the number of adjustment steps and the pulse count threshold values THRSHLD N  are determined based on characteristics of the particular print head  10  to provide the optimum print quality over the lifetime of the print head  10 .  
         [0044]    It is contemplated, and will be apparent to those skilled in the art from the preceding description and the accompanying drawings that modifications and/or changes may be made in the embodiments of the invention. Accordingly, it is expressly intended that the foregoing description and the accompanying drawings are illustrative of preferred embodiments only, not limiting thereto, and that the true spirit and scope of the present invention be determined by reference to the appended claims.