Abstract:
A system for regulating the vacuum hold pressure in a printer based upon the physical characteristics of the print media that is directed through the printer. In one embodiment, the characteristic of a sheet of media is detected before or as the sheet reaches the carrier. The vacuum pressure level is thus regulated in response to the physical characteristic of the sheet, thereby to have applied to that particular media a level of vacuum pressure that prevents problems that arise when pressure levels are too low (for example, inadvertent shifting of the paper) or too high (for example, paper deformations that reduce print quality).

Description:
TECHNICAL FIELD 
     This invention relates to systems that employ vacuum pressure for holding print media as the media is advanced through a hard copy device such as a printer. 
     BACKGROUND AND SUMMARY OF THE INVENTION 
     An inkjet printer includes one or more ink-filled pens that are mounted to a carriage in the printer body. Normally, the carriage is scanned across the width of the printer as paper or other print media is advanced through the printer. Each ink-filled pen includes a printhead that is driven to expel droplets of ink through an array of nozzles in the printhead toward the paper in the printer. The timing and nominal trajectory of the droplets are controlled to generate the desired text or image output and its associated quality. 
     As the sheet of print media is advanced through the printer, it must be secured so that the high-resolution printing can occur. One method of holding the sheet is to direct it against an outside surface of a moving carrier such as perforated drum. Suction is applied to the inside surface of the carrier for holding the sheet against the moving carrier. The carrier is arranged to move the sheet into and out of a location adjacent to the pens for receiving the ink. 
     It is important to apply the proper level of suction to a system like the one just described. The suction, or vacuum pressure (here the term “vacuum” is used in the sense of a pressure less than ambient), must be applied at a level sufficient for ensuring that the sheet of print media remains in contact with the carrier. Also, the level must be high enough to hold the sheet flat, to eliminate wrinkling or cockling of the sheet during printing. 
     If the vacuum pressure level is too high, the surface of the sheet may become deformed in the vicinity of the perforations. As a result, the ink droplets will not strike the surface of the sheet as intended, and print quality will suffer. Also, power is wasted if the vacuum level is unnecessarily high. 
     Moreover, when liquid ink is applied to the sheet, it is important to ensure that that vacuum pressure level is not so high as to draw the ink completely through the sheet, such that the ink appears on the other side as an undesirable effect known as “strike through.” 
     The foregoing considerations concerning vacuum levels are complicated by differences in the physical characteristics of the variety of print media that can be handled by modern printers. The print media can be thin, relatively lightweight cut paper, relatively thick or stiff media known as transparencies, heavy photo stock, etc. In short, one level of vacuum pressure will not be appropriate for the wide variety of print media available to a user. 
     The present invention is directed to a system for regulating the vacuum hold pressure in a printer based upon the physical characteristics of the print media that is directed through the printer. 
     In one preferred embodiment of the invention, the characteristic of the paper is detected before or as the paper reaches the carrier. The vacuum pressure level is thus regulated in response to the paper characteristic, thereby to have applied to that particular media a level of vacuum pressure that is best (remove cockle, avoid strike through, etc.) for that media. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a media carrier of a printer, which carrier is adaptable for use with the vacuum-hold regulating system of the present invention. 
     FIG. 2 is a side view of the media carrier, including media handling and sensing components of the present invention. 
     FIG. 3 is a block diagram of the present system. 
     FIG. 4 is a detail view of one preferred media-characteristic sensing apparatus in accord with the present invention. 
     FIG. 5 is a detail view of another preferred media-characteristic sensing apparatus in accord with the present invention. 
     FIG. 6 is a detail view of another preferred media-characteristic sensing apparatus in accord with the present invention. 
     FIGS. 7-9 depict the calibration and use of another media-characteristic sensing apparatus in accord with the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     With reference to FIGS. 1 and 2, a preferred embodiment of the present invention is operable with printer media carrier, such as a drum  20 , that is supported by a shaft  22  within a printer. The drum  20  preferably has a circumference of about 50 cm, although any of a variety of drum sizes will suffice. 
     An endless drive belt  24  engages a gear  28  that is fixed to one end of the drum  20 . That belt also engages a drive pulley  26  (FIG.  2 ). In a preferred embodiment, a motor (not shown) continuously drives the pulley  26  to rotate the drum whenever a printing operation is carried out. 
     The other end of the drum shaft  22  is hollow. A vacuum line  30  enters the hollow interior of the drum  20  through the shaft  22 . The other end of the vacuum line  30  is connected to a regulated vacuum system  35  (FIG.  3 ). The vacuum is applied to the interior of the drum as a mechanism for securing print medium, such as paper  32 , to the drum  20  as the paper is advanced through the printer over the drum. To this end, the drum is perforated with vacuum ports  34  that extend between the interior surface  25  of the drum and the outer surface  36  of the drum. The suction present in the ports  34  secures to the drum outer surface  36  the paper  32  that is directed into contact with the drum, as is described next. 
     FIG. 2 illustrates in somewhat simplified fashion a portion of the path of the paper  32  through the printer. It is noteworthy here that although “paper” will be hereafter referred to as the print medium, any of a number of materials can be used as the medium in such printers, such as thin, relatively lightweight cut paper, relatively thick or stiff media known as transparencies, heavy photo stock, etc. As will be described, the present invention provides for regulating the vacuum system  35  so that a level of suction is applied by the vacuum system to match the physical characteristics of the media. 
     The paper  32  is picked from an input tray and driven into the paper path in the direction of arrow  40 . The leading edge of the paper is fed into the nip between a drive roller  42  and an idler or pinch roller  44 . From there the paper  32  is driven in a controlled manner into contact with a curved guide  46  that, in cooperation with guide rods  48 , directs the leading edge of the paper  32  into tangential contact with the exterior surface  36  of the drum  20 . The guide rods are removed from contact with the paper as soon as the paper is loaded. 
     As the vacuum ports  34  of the drum rotate into contact with the paper  32 , the suction established between the paper and drum secures the paper to the drum, and the drum continues to rotate in the direction of arrow  50 . The paper  32  on the drum is advanced to be located adjacent to one or more pens  52  of the printer. The pens are controlled to apply ink to the paper during a printing operation. 
     Once the printing operation respecting a particular sheet of paper is complete (the paper may be rotated past the pens several times to complete the operation) the paper is removed from the drum. This can be carried out by the controlled, temporary movement of guide prongs  21  (FIG. 2) that pivot about a post  23  into a circumferential grooves  37  that are formed in the drum. This redirects the paper from the drum to a conveyor belt  39  that delivers the paper to a collection tray. 
     In one preferred embodiment of the present invention, the thickness characteristic of the paper  32  is detected as the paper approaches the drum  20 . To this end, a lever  54  is connected at one end to the shaft  56  of the pinch roller  44 . The lever is pivotally connected between its ends a pivot  58 , which is a fixed point relative to the printer. The remote end  60  of the lever has mounted to it an electrode  62  that faces another electrode  64  that is aligned with the first and is mounted to a fixed, electrically insulated pad  66  in the printer. 
     A deformable, conductive member  68  is located between and in contact with the two electrodes  62 ,  64 . The member  68  is made of conductive rubber in which the electrical conductivity changes in proportion to the pressure applied to it. In this regard, a low voltage is applied via lead  75  to the movable electrode  62  by the vacuum controller  80  (FIG.  3 ), discussed more below. Another lead  76  connects the fixed electrode  64  with the vacuum controller. Thus, the magnitude of the signal appearing on line  76  to the vacuum controller corresponds to that applied on line  75 , as affected by changes in the shape (i.e., conductivity) of the deformable member  68 . 
     As the leading edge  70  of a sheet of paper  32  passes between the drive roller  42  and the pinch roller  44 , the pinch roller  44  is lifted (arrow  72 ) by an amount corresponding to the thickness of the paper. As a result, the lever  54  pivots about point  58  such that the remote end  60  of the lever moves downwardly (arrow  74 ) and compresses the conductive member  68 . The attendant change in the conductivity of the member  68  varies the signal appearing on line  76  (hereafter referred to as the thickness signal) to the vacuum controller  80 . The location of the pivot  58  is selected to multiply the distance of roller  44  movement by an amount sufficient to provide measurable changes in the compression of the conductive member  68 . 
     The vacuum controller  80  monitors the thickness signal and adjusts the level of vacuum applied to the drum via line  30 . In this regard, the vacuum controller  80  may be incorporated into the overall printer controller and include suitable analog to digital converters for controlling the just described low-voltage circuit between it and the remote end of the lever  60 . 
     The vacuum controller  80  is also provided with suitable drivers for controlling via line  82  a conventional electronically controlled pneumatic valve  84 . The valve  84  is connected to the vacuum line  30 , which extends between a constant level vacuum source  88  and the drum  20 . The valve  84  is also interconnected between the line  30  and an atmospheric vent  90 . The valve is controlled by the controller  80  (as noted, in response to the thickness signal) to open the vent  90  by an amount sufficient to alter (lower) the vacuum pressure that in the line  30 , hence in the interior of the drum  20 . In this regard, the vacuum controller includes a look-up table or the like to correlate the thickness signal to the desired valve adjustment. This table can be empirically derived through tests of various media types. 
     One of ordinary skill will appreciate that there are many other ways available for adjusting the vacuum level applied to the drum. For instance, the source itself could be controllable (such as be varying fan speed) to increase or decrease the level as needed in response to the thickness signal. 
     FIG. 4 represents an alternative means for sensing the movement of the remote end  60  of lever, which movement, as explained above, relates to the thickness of the paper fed to the drum. In this embodiment, the remote end  60  of the lever has mounted to it an electrode  162  that faces and is spaced from another electrode  164  that is that is mounted to a fixed, electrically insulated pad  166  in the printer. Leads  175 ,  176  make a circuit as described above, except that the electrodes  162 ,  164  act as a capacitor. Accordingly, movement of the lever end (arrow  174 ) in response to the engagement of the roller  44  with the leading edge  70  of the paper, varies the capacitance across the two electrodes, which change is apparent on line  176 , which is received by the vacuum controller  80  as the thickness signal. The vacuum pressure level is then adjusted as needed, as discussed above. 
     It is contemplated that changes in inductance could be employed to sense movement of the lever end. For instance, the movable electrode  162  of FIG. 4 could be a ferromagnetic member moving relative to a coil, which would substitute for the fixed electrode  164 , the coil having current directed through it by the controller  80 . The inductance change attributable to the relative movement of ferromagnetic member and coil would alter accordingly the signal appearing on line  176 . It is also contemplated that the movement of an electrode (such as electrode  164 ) though a magnetic field could be sensed by an eddy-current proximity sensor that detects the eddy current changes in the electrode. 
     FIG. 5 represents an alternative means for sensing the movement of the remote end  60  of lever  54 . This embodiment uses an optical sensor  100 . Here, the end  60  of the lever is equipped with a plate  102 . The plate has a surface  104  that faces the emitter  106  (such as an infrared emitter) and detector  108  (such as a photodiode) of the optical sensor  100 . The surface  104  is coated with reflective material in a pattern where the width of the material, hence the intensity of the emitter light reflected back to the detector, varies in the direction of movement of the lever end  60  (up and down in FIG.  5 ). As a result, the output from the sensor  100  that is applied via line  276  to the vacuum controller (the thickness signal) varies with the lever movement, which, as described, relates to the paper thickness, which in turn correlates to a preferred vacuum pressure level to be applied to the drum interior. It will be appreciated that many other optical-type sensors can be used to detect and quantify motion of the lever end. 
     The various sensors described herein can be calibrated in a number of ways. For instance, one could configure the roller  44  with a known runout, and as the roller turns, the variation of the signal will indicate the output change of the sensor associated with a position change of the roller that corresponds to the runout. Alternatively, such calibration (and subsequent sensing) can be made by replacing roller  44  with a pin that rides in a predetermined, variable-depth notch formed in roller  42 . 
     FIG. 6 shows another alternative means for sensing the movement of the remote end  60  of lever  54 . In this case, the characteristic of the paper that is detected can be considered as the stiffness of the paper. That is, two different papers having the same thickness may have different stiffnesses (resistance to bending). Moreover, such papers, owing to the difference in stiffness, may require different vacuum holding pressures to avoid the problems discussed above. 
     The embodiment of FIG. 6, which can be used alone or in conjunction with the thickness detection approaches discussed above, provides the vacuum controller  80  with a measure of the paper&#39;s stiffness (hereafter referred to as the stiffness signal) so that the vacuum pressure level can be adjusted accordingly (using, for example, a look-up table relating the stiffness signal to the desired vacuum level). 
     In FIG. 6, the leading end of the paper  70  is directed into the path of a curved guide  120 . In the absence of any appreciable paper stiffness, the guide  120 , which is carried on the end of a lever  121  and urged by a spring  122  toward a feed roller  124 , would immediately bend the paper into a desired path, shown as arrow  126 . 
     In the event the paper has an appreciable amount of stiffness, the initial contact between the leading edge  70  (note dashed lines for  70 ) and guide  120  will deflect the guide slightly, thereby causing, at least momentarily, the lever  121  to pivot about its pivot point  178 . The lever is configured so that this pivot motion moves its remote end  130  in a manner that can be detected by any of the sensing mechanisms described above, so that the vacuum pressure level can be adjusted accordingly. 
     Although preferred and alternative embodiments of the present invention have been described, it will be appreciated by one of ordinary skill that the spirit and scope of the invention is not limited to those embodiments, but extend to the various modifications and equivalents as defined in the appended claims. 
     For example, there may be fewer or more perforations in the drum as compared to what is depicted. Also, the drum need not be a rigid, cylindrical member. For instance, the drum may be more like a porous conveyor belt of any given configuration. 
     Further, although a lever was primarily described in the foregoing, it will be clear that any member that engages the paper, the movement of which member can be sensed, may be used to detect the paper characteristics. 
     Also, other media characteristics may be detected, such as porosity, in the course of applying the most appropriate vacuum pressure level. One preferred embodiment of a media-porosity sensing system is shown in FIGS. 7-9. 
     The porosity sensor includes a head  200  that defines a substantially closed chamber  202  that underlies the path of the paper  32  as the paper moves toward the drum  20 . The head  20  includes a flat contact surface  204  across which the paper  32  is guided (See FIG.  9 ). The contact surface is interrupted with a slot  206  that is defined by side walls  208  that extend from the surface  204  into the chamber  202 . 
     The chamber  202  is connected by a conduit  210  to a constant-level vacuum source. In this regard, the conduit  210  may be connected to the above-described source  88 , preferably in a manner that ensures constant vacuum pressure in conduit  210 . 
     A lever-like valve  212  is pivotally mounted (as at pivot  214 ) in the chamber  202 . The free end  215  of the valve is connected to a spring  216  that normally urges the valve into a closed position (See FIG. 8) so that the valve  212  is seated against the innermost ends of the slot side walls  208 , thereby to occlude fluid communication between the chamber  202  and the slot  206 . 
     The characteristics of the spring  216  are selected so that whenever the slot  206  is obstructed at the contact surface  204  (that is, so that no air is free to move into the chamber  202 ), the levered valve  212  will move into the closed position (FIG.  8 ). When the slot  206  is not obstructed, the vacuum pressure in the chamber is sufficient to deflect the valve  212  from the closed position, thus extending the spring  216  (FIG.  7 ). 
     It will be appreciated that the movement of the free end  215  of the levered valve  212  can be sensed by any of a number of techniques, such as those described above. Moreover, the amount of deflection of the valve  212  (that is, between the closed position of FIG.  8  and the completely open position of FIG. 7) will vary depending on the porosity of the material, such as paper  32 , that is directed over the slot  206 . 
     The apparatus just described is first calibrated by sensing the position of the end  215  when the slot  206  is unobstructed. This deflection is shown by the angle θ 0  in FIG.  7 . In addition to controlling the vacuum pressure in the chamber  202 , and the characteristics of the spring  216 , the preferred maximum amount of end  215  deflection can be controlled by providing the slot  206  with flow restricting material, such as foam  218 . The maximum deflection θ 0  is established to be greater than would occur if the slot were covered with the most porous print media available. 
     A non-porous obstruction  220  may then be placed over the slot  206  to establish for calibration the precise position of the valve end  215  when the valve is in the closed position, shown as angle θ c  in FIG. 8 (0 degrees). Thereafter, the apparatus is used by directing paper  32  in the direction shown by arrow  40 . Depending on the porosity of the paper (which, as noted can be any print media), the lever  212  will deflect by an amount θ m  that is sensed and, as discussed above, correlated to a preferred vacuum pressure level to be applied to hold the paper to the drum  20 .