Abstract:
The present disclosure describes a system and method for controlling the post-marking heaters and ink spreading nip rollers employed for spreading the ink dots on the media web to provide the desired image quality. The technique employs a sensor for sensing single pixel linewidth and showthru sensor to sense showthru of the ink. The sensor outputs are combined with the temperature and pressure values of the ink spreading nips to provide an error signal to a PID controller for controlling the nip pressure and heater temperature.

Description:
BACKGROUND 
       [0001]    The present disclosure relates to solid ink jet marking of dots on continuous web print media from a digital image. In the aforesaid process, it is necessary to provide accurate control of spreading of the dots on the image page in order to obtain good print quality by masking eventual missing jets and improving the robustness of the image. The process includes solid ink jet marking units and heaters for heating the ink of the marked image prior to entry into spreading nip rollers for providing the desired spreading of the ink dots to give a quality image, particularly where the image is marked with multiple colorant inks. Heretofore, problems have been encountered in maintaining the quality of the image on the marked side of the web media and in controlling the showthru properties of the ink where the web is to be also marked on opposite sides thereof with desired images. It has been found difficult to control the effect of the heaters and nip pressure in the process to provide the desired quality of the marked images on the web; and, thus it has been desired to provide an improved way or means of controlling these functions for quality printing. 
       BRIEF DESCRIPTION 
       [0002]    The present disclosure describes a system and method for controlling a dot spreading subsystem employed for spreading ink dots on the media web to provide desired image quality. The subsystem consists of post-marking heaters and ink spreading nip rollers, and uses optical array sensors for sensing single pixel linewidth and show thru of ink dots in the media, and an optical array sensor, dubbed image-on-web array (IOWA) sensor, for sensing images in the process and cross-process direction. The sensor outputs are integrated with the temperature and pressure readings of the ink spreading nips to provide an error signal to a PID controller for controlling the nip pressure and temperature and the heater temperature. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0003]      FIG. 1  is a pictorial of the system architecture employed in the present disclosure; 
           [0004]      FIG. 2  is an enlarged view of a portion of  FIG. 1  illustrating the spreading subsystem; 
           [0005]      FIG. 3  is a graphical presentation of pixel linewidth as a function of the spreader subsystem set points; 
           [0006]      FIG. 4  is a control diagram of the operation of the control of the spreader subsystem; and 
           [0007]      FIG. 5  is a block flow diagram of the control arrangement for the image sensors, spreader and heaters of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0008]    Referring to  FIG. 1 , a schematic diagram of the multi-colorant printing system architecture is shown wherein the system indicated generally at  10  includes a web module  12  to which the continuous media web  14  passes for marking. The module  12  includes the control and motor drives for movement and tension control of the web. For example, a load cell  40  and encoders  50  and  60  are used for sensing tension and speed in the web, respectively. The printing system  10  includes two print modules  18  and  20 , furnished with print units each having several printheads to enable wide image printing. In this embodiment of the disclosure, the print units are indicated by reference numerals  20 ,  22 ,  24 ,  26 ,  28 ,  30 ,  32 ,  34 ,  38 ,  40 , and  42  for enabling different configurations of printing, including monochrome, CMYK, and hexachrome ink printing. 
         [0009]    A machine controller  46  shown in  FIG. 1  receives video data from the digital front end  44  and is in operational control of the web driver module, feeding media, adjusting web velocity and tension, of the printing modules and spreading subsystem, and any other subsystem required for enabling printing. 
         [0010]    The system  10  of  FIG. 1  includes a web cleaning brush  36 , contact nip rollers  38  and a load cell  40  for providing a signal to aid in tension control and a pre-heater roll  48  with encoder indicated generally at  50 . The preheat roll is a web driving roll that also controls the temperature of the media entering the printing zone. Media temperature usually ranges between 25 C and 75 C, depending on the media properties. In the printing zone ink dots are jetted from the print heads at temperatures ranging between 110 C and 140 C, depending on the ink properties, and deposited on the media. After exiting the printing zone the ink image on media enters a leveler roll  58  whose function is to equate the ink-media by cooling. In another embodiment the leveler function is not required, as the ink-media enters directly into the spreading subsystem. 
         [0011]    Referring to  FIGS. 1 and 2 , the web  14  is shown entering an image sensor system which includes a sensor for sensing the image on the web  62  and a backer roll  64 , denoted by the reference characters IOWA in the drawings, and which is disposed downstream of the leveler  58  in the process direction for sensing the presence and correctness of the image marked on the web  14 . The marked image on the web then enter the spreader module  68 , first passing through a heater array indicated generally at  66  to adjust the ink-media temperature. The spreader module  68  includes a spreader drum  74 , a pressure roll  72  and an oiler module  70 , for spreading the ink drops on the web to achieve the desired image quality. In another embodiment either leveler  58  and or heater  66  are not employed, and the web  14  passes directly from the print zone  18  to the spreading rolls. 
         [0012]    A linewidth sensor  76  and a showthru sensor  78  are provided adjacent opposite sides of the web downstream of the spreader module  68  in the process direction. The linewidth sensor senses the width of a single pixel line; and, the showthru sensor detects the ink bleed through the media to the opposite side. 
         [0013]    The IOWA sensor is disposed upstream of the heater  66  and is operative to detect the width of a pixel line prior to entry into the spreader module. Thus, by comparison of the linewidth measurements in sensor  66  and sensor  76 , the change in linewidth due to the effects of the operations in the spreader module  68  can be measured; and, from that relationship appropriate control algorithms may be applied for control of temperature and pressure applied to the web  14 . 
         [0014]      FIG. 3  illustrates the functional dependence of single pixel linewidth in microns as a function of the paper temperature in degrees centigrade and spreader roll pressure in psi. The paper stock weight used in these measurements was 75 gsm.  FIG. 3  shows the functional dependence of temperature and pressure on line width for this particular ink. It also indicates the values of these parameters where the show-thru threshold is reached. Beyond these values, duplex print image quality degrades to unacceptable values.  FIG. 3  provides the necessary information to construct the transfer function (such as a Jacobean transform) between the control parameters and the line width output, and to thereby construct a PID controller. 
         [0015]    Referring to  FIG. 4 , a generic PID controller for the spreader module  68  is shown diagrammatically wherein the control reference signal on line  80  is summed at input junction  81  with an error feed-back signal along line  92  to output summing junction  84 . The error signal from line  92  and the reference input signal  80  are applied along  84  to a proportional-integral-derivative controller  86  which outputs to the actuators at plant  88 , for nip pressure and temperature. The linewidth sensor  76  and the showthru sensor  78  measure the control target on the web at the output of the block  88  and provide an integrated signal to the IOWA summing junction for computing the error and thus close the loop. 
         [0016]    In the present practice, it has been found satisfactory to utilize a full width optical array sensor for the linewidth sensor  76  and showthru sensor  78  with a cross process resolution of 600 spots per inch or 42.3 micrometers. The IOWA sensor  62  in the present practice has a 600 spots per inch resolution. Then, after combining with appropriate signal processing techniques and statistical sampling, the sensing system provides linewidth measurements with a resolution of less than 3 micrometers (3σ). The full width array capability of this embodiment allows the determination of inboard/outboard non-uniformities of the output of the spreader module whereupon the correction includes inboard/outboard differential actuation. 
         [0017]    Referring to  FIG. 5 , a block diagram of the control arrangement for the spreader module and sensors is shown in which the sensors  76 ,  78  have their output signals processed in a signal processing step  92  and the signals proceed to step  94  wherein control and actuator signals are generated and the outputs provided to the spreader  74  and the heater  66 . The sensors  76 ,  78 , spreader  74  and heater  66  are shown as disposed in a full width disposition in the cross-process direction with respect to the movement of the web as shown by the black arrow in  FIG. 5 . In another embodiment, standard linear optical array sensors of reduced sensing width, e.g., approximately 0.5 inches long, can be used for the linewidth  76  and showthru  78  sensors, or alternatively, to also address inboard/outboard nonuniformities, a multiplicity of them positioned across the web width can be used. 
         [0018]    It will be appreciated that several of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.