Abstract:
A method for heating a substrate in a printing device, the method comprises positioning at least one radiant heater along a printing path of a printing device, the at least one radiant heater includes at least two emitters; measuring a voltage and current supplied to each of the at least two emitters; determining an electrical power supplied to each of the at least two emitters; and adjusting the electrical power supplied to at least one of the at least two emitters if a difference in power supplied to each of the at least two emitters exceeds a threshold.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Reference is made to commonly-assigned, co-pending U.S. patent application Ser. No. ______ (Kodak Docket K001796US01) filed concurrently herewith, entitled “A DRYER FOR HEATING A SUBSTRATE”, by Rodney R. Bucks, et al the disclosure of which is incorporated herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention generally relates to dryers for continuous inkjet printers and more particularly to a method for more uniformly drying print media passing through printers. 
       BACKGROUND OF THE INVENTION 
       [0003]    In a digitally controlled inkjet printing system, a receiver medium (also referred to as a print medium) is conveyed past a series of components. The receiver medium can be a cut sheet of a receiver medium or a continuous web of a receiver medium. A web or cut sheet transport system physically moves the receiver medium through the printing system. As the receiver medium moves through the printing system, liquid (e.g., ink) is applied to the receiver medium by one or more printheads through a process commonly referred to as jetting of the liquid. The jetting of liquid onto the receiver medium introduces significant moisture content to the receiver medium, particularly when the system is used to print multiple colors on a receiver medium. Dryers are then used to remove moisture from the receiver medium. 
         [0004]    Although the prior art methods are satisfactory, they include drawbacks. Due to aging and the like, the heating elements within the dryer do not heat uniformly. Consequently a need exists for more uniform heating within the dryer so that the print medium passing through it is uniformly heated. 
       SUMMARY OF THE INVENTION 
       [0005]    The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the invention, the invention resides in a method for heating a substrate in a printing device, the method comprises positioning at least one radiant heater along a printing path of a printing device, the at least one radiant heater includes at least two emitters; measuring a voltage and current supplied to each of the at least two emitters; determining an electrical power supplied to each of the at least two emitters; and adjusting the electrical power supplied to at least one of the at least two emitters if a difference in power supplied to each of the at least two emitters exceeds a threshold. 
         [0006]    These and other objects, features, and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter of the present invention, it is believed that the invention will be better understood from the following description when taken in conjunction with the accompanying drawings, wherein: 
           [0008]      FIG. 1  is a schematic side view of a digital printing system for continuous web printing on a print medium; 
           [0009]      FIG. 2  is a schematic side view of components in a portion of the digital printing system; 
           [0010]      FIG. 3  is a top view of the dryer of  FIG. 2  illustrating the emitters within the dryer; and 
           [0011]      FIG. 4  is a schematic diagram of the dryer. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    Referring to  FIG. 1 , there is shown a digital printing system  5  for continuous web printing on a print medium  10 . The digital printing system  5  includes a first module  15  and a second module  20 , each of which includes lineheads  25 - 1 - 25 - 4 , dryers  40 , and a quality control sensor  52 . In addition, the first module  15  and the second module  20  include a web tension system (not shown) that serves to physically move the print medium  10  through the digital printing system  5  in the in-track direction  12  (left to right as shown in the figure). 
         [0013]    The print medium  10  enters the first module  15 , from the source roll (not shown). The linehead(s)  25 - 1 - 25 - 4  of the first module applies ink to one side of the print medium  10 . As the print medium  10  feeds into the second module  20 , there is a turnover mechanism  50  which inverts the print medium  10  so that linehead(s)  25 - 1 - 25 - 4  of the second module  20  can apply ink to the other side of the print medium  10 . The print medium  10  then exits the second module  20  and is collected by a print medium receiving unit (not shown). For descriptive purposes only, the lineheads are labeled a first linehead  25 - 1 , a second linehead  25 - 2 , a third linehead  25 - 3 , and a fourth linehead  25 - 4 . 
         [0014]    Referring to  FIG. 2 , a portion of the digital printing system  5  is shown in more detail. As the print medium  10  is directed through the digital printing system  5 , the lineheads  25 - 1 - 25 - 4 , which typically include a plurality of printheads  32 , apply ink or another liquid, via the nozzle arrays  34  of the printheads  32 . The printheads  32  within the lineheads  25 - 1 - 25 - 4  are located and aligned by a support structure  30 . After the ink is jetted onto the print medium  10 , the print medium  10  passes beneath the one or more dryers  40  which apply heat to the ink on the print medium  10 . The applied heat accelerates the evaporation of the water or other solvents in the ink. The dryer  40  is preferably a radiant heater  42  and includes a plurality of emitters  45  which generates the heat for drying the print medium  10 . Referring to both  FIGS. 2 and 3 , the emitters  45  (labeled  45   a - 45   d  in  FIG. 3 ) are preferably positioned in the in-track direction  12 , the direction of the flow of the print medium  10 . It is noted that positioning the emitters  45   a - 45   d  ( FIG. 3 ) in the in-track direction  12  provides the advantage of permitting the outer emitters  45   a  and  45   d  to be turned completely off if the print medium  10  is narrower than the width of the emitter array. While the dryers of  FIGS. 2 &amp; 3  include four emitters  45   a - 45   d,  the invention is applicable to dryers  40  having two or more emitters  45 . A temperature sensor  60  is positioned adjacent the radiant heater  42  for measuring the temperature of the web after exiting the dryer  40 . Preferably, a single temperature sensor  60  is associated with each dryer  40 , and is typically positioned downstream of the radiant heater  42 . The emitters  45  are preferably carbon, tungsten halogen, or quartz emitters operating at a color temperature of between 3000K and 700K. Although only one dryer  40  is shown in  FIG. 2 , a plurality of dryers  40  is typically used as shown in  FIG. 1 . It has been found that heat applied to the web of print medium  10  by the different emitters  45  can vary significantly even when they are nominally the same. As a result, the temperature of the print medium  10  as it leaves a dryer  40  can vary significantly across the width of the dryer  40 . Excessive temperature differences across the print medium  10  can cause either or both some portions to be insufficiently dried or some portions of the print medium  10  to become sufficiently hot that there can be an increased risk of moisture condensing onto printer components downstream of the dryer  40 . Referring to  FIG. 4 , the four emitters  45   a - 45   d  are each respectively connected to its associated circuitry. For simplicity of discussion, only one emitter  45   a  and its associated circuit will be discussed in detail while it is noted that each emitter  45   b - 45   d  includes the same associated circuits, for example voltage sources  55   b - 55   d,  volt meters  70   b - 70   d  and amp meters  65   b - 65   d  respectively. In this regard, emitter  45   a  includes a voltage source  55   a  and an amp meter  65   a  connected in series to the emitter  45   a.  The voltage source  55   a  provides the electrical current for energizing the emitter  45   a,  and the amp meter  65   a  measures the amount of current flowing through the circuit. A volt meter  70   a  measures the voltage across voltage source  55   a.  When energized, the emitter  45   a  generates radiant heat for heating the print medium  10 , and the amp meter  65   a  and volt meter  70   a  respectively monitor the current and voltage. A controller  75  receives a signal from both the volt meter  70   a  and amp meter  65   a  and uses this information to calculate the electrical power for this particular circuit as is well known in the art. In general, the impedance of the emitters  45   a - 45   d  is primarily resistive, so that the voltage and current are in phase with each other and the electrical power supplied to the emitters  45   a - 45   d  is the product of the voltage and the current. If emitters  45   a - 45   d  are used that have a significant capacitance or inductance, a phase meter can also be used to measure the phase between the voltage and current so that the real portion of the electrical power supplied to the emitters  45   a - 45   d  can be determined. If it is determined that one emitter  45  is receiving more electrical power than a second emitter  45 , by more than some defined threshold amount such as 3% more, then the controller  75  compensates for this by adjusting the voltage, and therefore the electrical power to at least one of the two emitters  45  to a desired balance of power. The collective power output of the dryer  40  is the sum of the outputs of each of the individual emitters  45   a - 45   d.  There is a dryer power output setting that is used to control the collective power delivered by all of the emitters  45   a - 45   d  in the dryer  40 . A target power value for the individual emitter circuits can simply be determined by dividing the dryer target power value by the number of emitters  45  in the dryer hereinafter called the emitter power target value. If the individual emitter circuit calculated power differs from the emitter target power value by more a threshold value, preferably equal to or greater than 3%, the controller sends a signal to the voltage source  55  to adjust its output accordingly so that the power output of all emitters  45   a - 45   d  is substantially equal and so that the power output of the emitters  45   a - 45   d  collectively matches the dryer power target value. For example, if the emitter target power value is 1000 watts, 1030 watts or greater or 970 watts or less would trigger the adjustment. The dryer target power value is determined by the controller  75  typically in response to the print speed of the printer and to a setting provided by the printer operator or determined by a controller  75 . In regard to operator control, the operator may observe some characteristics of the print medium  10  or some aspect of the digital printing system  5  and alter the power settings of the dryer  40 . In regard to the determination by the controller  75 , a target temperature is predetermined from prior knowledge of the digital printing system  5  or the print medium characteristics. The temperature sensor  60  ( FIG. 3 ) provides temperature feedback to the controller  75 . The controller  75  then adjusts the power settings of the dryer  40  until the target temperature is achieved. The setting of the dryer target power values by the controller  75  as described above is one example of how the dryer target value may be determined. 
         [0015]    In an alternative embodiment, the emitter target power value is set to be equal to the measured power of a reference emitter  45 , for example emitter  45   b.  The reference emitter  45  is preferably in line with the temperature sensor  60  as illustrated in  FIG. 3 . In this case, a dryer target power value is not needed. 
         [0016]    The above description applies to the emitters  45   b - 45   d  so that the controller  75  is permitted to monitor and adjust the output of each emitter  45   a - 45   d  as determined by the target power value and the allowed emitter power variation. 
         [0017]    This provides improved radiant energy uniformity by adjusting the supplied voltage to each emitter  45   a - 45   d  so that the electrical power of each emitter  45   a - 45   d  is the same. A significant reduction in emitter energy output variability and an improvement in delivered energy uniformity are achieved by monitoring the RMS (root mean square) voltage supplied to each emitter  45   a - 45   d  and the RMS current passing through each emitter  45   a - 45   d,  when compared to prior art systems that supplied a uniform supply voltage to each of the emitters  45   a - 45   d.    
         [0018]    The invention provides better control in variable data printing systems than does a system that constantly monitors the temperature uniformity across the width of the print medium  10 , and varies the power delivered to the various emitters  45   a - 45   d  in response to that measured temperature uniformity. This is due to the variability, both spatially across the web and over time, of ink applied to the print medium  10 . The varying amounts of ink applied, as it is evaporated from the print medium  10  in the dryer  40 , provide varying amounts of evaporative cooling to the print medium  10 . Such varying amounts of web cooling can cause dryer control systems that try to maintain a uniform temperature across the print medium  10  to operate erratically. The present invention avoids such problems by monitoring the electrical power supplied to each emitter  45   a - 45   d  and adjusting the supply voltage to the various emitters  45   a - 45   d  to produce the desired balance of supplied power. 
         [0019]    The controller  75  receives voltage and current measurements from the volt meters  70   a - 70   d  and amp meters  65   a - 65   d  associated with each of the dryer emitters  45 . In some embodiments of the invention, through monitoring and analysis of these measurements, the controller  75  can detect early signs of an impending emitter  45  failure. The controller  75  can then provide a warning to the operator of the impending failure so that the failing emitter  45  can be replaced. 
         [0020]    The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
       PARTS LIST 
       [0000]    
       
           5  Digital printing system 
           10  Print medium 
           12  In-track direction 
           15  First module 
           20  Second module 
           25 - 1  first Linehead 
           25 - 2  Second Linehead 
           25 - 3  Third Linehead 
           25 - 4  Fourth Linehead 
           30  Support Structure 
           32  Printheads 
           34  Nozzle arrays 
           40  Dryers 
           42  Radiant Heater 
           45  Emitter 
           45   a - 45   d  Emitters 
           50  Turnover Mechanism 
           55  Voltage Source 
           55   a - 55   d  Voltage Sources 
           60  Temperature Sensor 
           65   a - 65   d  Amp Meters 
           70   a - 70   d  Volt Meters 
           75  Controller