Abstract:
A system for regulating the vacuum hold pressure in a printer based upon the stiffness of the print media that is directed through the printer. In one embodiment, the stiffness 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 stiffness measurement 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). A sensing technique for alternatively sensing stiffness and paper thickness is also provided.

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 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 that apply the ink to the sheet. 
     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. For example, should the edges of the sheet lift from the carrier as a result of too little vacuum pressure, there is a likelihood that the pen will collide with the edge, which is quite undesirable. Also, the vacuum pressure 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 the 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. Also, media having the same thickness will not necessarily deform by the same amount for a given vacuum pressure level. For example, a sheet of transparency type media having a given thickness will not deform by the same amount as a sheet of paper having the same thickness. 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 controlling or regulating the vacuum hold pressure in a printer based upon the sensed stiffness of the print media that is directed through the printer. 
     In one preferred embodiment of the invention, the deflection of the print media is sensed before or as the media reaches the carrier. The vacuum pressure level is regulated in response to this deflection measure, 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 print media carrier of a printer, which carrier is adaptable for use with the vacuum-hold control 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 an enlarged view of a media stiffness sensing station component of the present invention. 
     FIG. 4 is a block diagram of the present system. 
     FIG. 5 is a side view of the media carrier depicting an another preferred embodiment of a media stiffness sensing station of the present invention. 
     FIG. 6 is a diagram of another preferred embodiment of a media stiffness sensing technique in accord with the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     With reference to FIGS. 1 and 2, one preferred embodiment of the present invention is operable with a 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 shaft has openings inside the drum to enable fluid communication between the end of the vacuum line and the drum interior. The other end of the vacuum line  30  is connected to a regulated vacuum system  35  (FIG.  4 ). 
     The vacuum is applied to the interior of the drum as a mechanism for securing print media, such as a sheet of 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 the print medium will be hereafter referred to as “paper”, 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 paper; namely, the stiffness of the paper. 
     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 . Upon exiting the rollers  42 ,  44 , the paper moves across a station  41  that senses the stiffness of the paper as described more fully below. 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 . 
     As the vacuum ports  34  of the drum rotate into contact with the paper  32 , the suction established between the paper and drum secures (“loads”) the paper to the drum, and the drum continues to rotate in the direction of arrow  50 . The guide rods are retracted from contact with the paper as soon as the paper is loaded. The paper  32  on the drum is moved to a location 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 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 of the paper  32  may be detected just before the paper  32  reaches the station  41 . To this end, a lever  54  is connected at one end to the shaft  56  of the pinch roller  44 . The lever has 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 electrode  62  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.  4 ). The controller is 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  rotates about pivot  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 . 
     As the paper  32  passes across the station  41  a measure of its stiffness is made and provided to the vacuum controller  80 . In this regard, the paper is deliberately deflected at the station  41  and a non-contact-type sensor  43  senses the amount of deflection. The particulars of one embodiment of the station  41  are best considered in connection with the detail view of FIG.  3 . 
     The paper  32  is moved across the surface  45  of a platform  47  that makes up part of the station  41 . The platform is the upper wall of an elongated, substantially hollow, bar-like member that extends across the width of the paper, perpendicular to the path of the paper. The underside of the station has at least one tubular stub  49  (FIG. 2) that is in fluid communication with the hollow interior of the station. The other end of the stub  49  is connected to a vacuum line  51  that connects (FIG. 4) with a constant level vacuum source that is discussed more below. 
     The vacuum pressure in the station  41  is communicated via one or more ports  53  to a channel  55  that is recessed in the surface  45  of the platform  47  (FIG.  3 ). The channel can be any of a variety of shapes, and need not extend across the width of the paper. In one preferred embodiment, the channel is generally rectangular, with its long sides extending in a direction parallel to the width of the paper  32  (i.e., into the plane of FIG.  3 ). The depth of the channel  55  (that is, the depth of the recess from the surface  45 ) is preferably about 10 to 12 mm. As noted, many other channel sizes will suffice for permitting deflection of at least part of the paper. 
     While part of the advancing paper  32  spans the channel  55 , the suction in the channel causes the paper to deflect from the generally planar orientation (shown as dashed line  57 ) it would assume in the absence of the applied suction. The sensor  43  is located adjacent to the channel  55  and senses the amount of deflection of the part of the paper that spans the channel. 
     In particular, the non-contact type sensor  43  may be an optical type, including a light emitter  59  and an array of light detectors  61 . The emitter  59  and detectors  61  are spaced apart. Light is directed via a beam  63  to be incident on the center of the channel, thereby to be reflected from the paper  32 . Depending upon the amount of deflection of the paper  32 , different ones of the array of detectors  61  will receive different amounts of light reflected from the paper. (Alternatively, a single light detector would receive a different amount of light, depending upon the amount of paper deflection.) 
     For instance, when the paper is deflected as shown in FIG. 3, the reflected light beam  65  strikes a different area in the light detector array than does a beam  67  that is reflected from a non-deflected surface  57  of the paper  32 . Thus, different values of an output signal (hereafter referred to as the stiffness signal) will appear on the output lead  69  of the sensor  43 , which signal is provided to the vacuum controller  80 . 
     The vacuum controller  80  monitors the stiffness signal, as well as the above-described 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 receiving and processing the just described stiffness and thickness signals. 
     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  that 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 and stiffness signal) to open the vent  90  by an amount sufficient to alter (lower) the vacuum pressure 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 stiffness signal and 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 vacuum source itself could be controllable (such as be varying fan speed) to increase or decrease the level as needed in response to the stiffness signal and the thickness signal. 
     Although separate thickness measuring mechanisms (lever  54 , electrodes  62 ,  64  etc.) were described earlier, another preferred embodiment of the present invention employs the components associated with the station  41  to serve a dual purpose of measuring thickness of the paper  32  in conjunction with the stiffness (i.e., deflection) of the paper, thereby eliminating the need for separate mechanisms for measuring these two paper characteristics. 
     With particular reference to FIG. 3, the thickness of the paper  32  can be measured by the sensor  43  while no vacuum pressure is applied to the channel  55  (hence the uppermost surface of the paper  32  takes the planar orientation shown at  57 ). In this regard, the sensor may be first calibrated in any of a number of ways, such as by sensing the non-deflected thickness of a paper having a known thickness. 
     Calibration of the sensor is not required. That is, a true thickness measure is not required for the vacuum control aspects of the present invention. The comparison of the measurements of the non-deflected paper surface (vacuum off) and of the deflected surface (vacuum applied) suffices for controlling the vacuum level. Thus, even though the sensor output signal corresponding to the non-deflected paper surface is characterized as a “thickness” signal, one will appreciate that the thickness of the paper need not, in fact, be determined to implement this embodiment of the present invention. 
     As mentioned, the deflection of the paper is measured after the vacuum pressure is applied (via the source  88  through vacuum line  51 ) to the channel  55 . In short, the vacuum pressure is cycled off and on during the time a particular sheet of paper spans the channel  55 . To this end, the vacuum line  51  that extends from the vacuum source to the station is equipped with a electronically controlled valve  81  (FIG. 4) that is opened and closed by the vacuum controller  80  via control line  83 . The valve  81  vents the line  51  to atmosphere when that valve is closed. Suction is applied to the line (hence to the channel  55 ) when the valve is opened. 
     Thus, for a given sheet of paper, a reflected light beam  67  (refer to FIG. 3) received on the detectors of the array  61  while the vacuum pressure in the channel is absent (i.e., valve  81  is closed) represents the thickness (or location of the non-deflected surface) of the paper, and a reflected light beam  65  received on the detectors of the array  61  while the vacuum pressure in the channel is present (i.e., valve  81  is open) represents the deflection of the paper. Therefore, the signal appearing on the sensor output lead  69  varies in time between the above defined thickness signal and stiffness signal. 
     It will be appreciated that, as another alternative, a separate, non-contact type sensor  43  could be employed as a paper thickness-measuring sensor. That is, a sensor otherwise like that  43  shown in FIG. 3 could be located adjacent to the platform  47  away from the channel  55  so that the part of the paper  32  underlying that second sensor remains supported on the surface  45  of the platform. As a result, only a paper thickness signal is generated by that second sensor. 
     FIG. 5 represents an alternative approach to the present invention whereby the deflection station is incorporated into the paper carrier component of the system. In this embodiment, the carrier is a drum  120  in which one of the flat end walls  85  is stationary. The curved surface  136  of the drum is sealed to but rotatable around the periphery of that wall  85 . The other end wall of the drum is connected to a shaft and pulley arrangement for rotating the drum in a manner as described above. 
     A pair of partitions  87  divides the interior of the drum  120  into two sectors. The partitions are rigid plates that are fixed to the end wall  85 . The inner radial edges of the partitions are formed of resilient, low friction material that makes a sealing engagement with, and slides along, the rotating shaft  122 . The outer radial edges of the partitions similarly slide against the inner surface  125  of the carrier  120 . 
     An inlet port  89  extends through the end wall  85  between the partitions  87 . That port  89  is connected with the vacuum line  51  (FIG. 4.) As a result, the sector  91  of the drum interior is provided with a relatively high level of suction that, as will be explained, is useful for both loading the paper  132  onto the drum  120  and for measuring the thickness and stiffness of that paper. 
     In response to the thickness and stiffness measure, the controller  80  controls the vacuum applied to the other sector  93  of the drum interior. That other sector  93  is the one underlying the pens  152  of the printer and, therefore, the vacuum level within that sector  93  is controlled or regulated to remain within the desired range discussed above for removing cockle, avoiding strike through, etc., for the particular sheet of paper  132 . 
     As shown in FIG. 5, the surface  136  of the drum  120  is provided with groups of ports and channels  155  that substantially match the port  53  and channel  55  made in the platform  47  of the previously described station  41  (FIG.  3 ). In one preferred embodiment, groups of the ports and channels  155  are spaced apart and distributed evenly around the surface  136  of the drum. 
     A sensor  143  that substantially matches the earlier-described sensor  43  is mounted to the printer and located outside the high-vacuum sector  91 . The light emitter of this sensor is directed toward the drum surface and, therefore, applies on its output a stiffness signal whenever a channel  155  (with paper spanning the channel) passes next to the sensor  143 . 
     The controller  80  receives the stiffness signal from the sensor  143 . Also as the channel  155  is rotated away from the sensor  143 , the beam emitted by the sensor strikes a non-deflected part of the paper  132 . Thus, the sensor output alternates between the stiffness signal and the thickness signal. For a given sheet of paper, the controller treats the received signal having the greater magnitude (most deflection) as the stiffness signal and the other as the thickness signal. In instances where no paper is carried by the drum, the signal returned by the sensor is outside of a predetermined threshold (which threshold is established by a calibration process using an empty drum) and the signal is ignored. 
     The stiffness and thickness signals are then processed as described above to control the vacuum level in the sector  93  of the drum underlying the pen  152 . In this regard an inlet  101  is provided through the end wall  85  and connected to the vacuum line  30  (FIG.  4 ). The suction level in sector  93 , therefore, is controlled in a like manner as that of the drum interior as described above in connection with the earlier embodiment. 
     It will be appreciated that in the just described embodiment (FIG. 5) the high level of vacuum pressure applied to the sector  91  where the paper  132  is first brought into contact with the drum  120  is useful for ensuring that the paper is properly drawn (loaded) onto the drum. This is in addition to the use of the high-level vacuum pressure for deliberately deflecting the paper to obtain the stiffness measure. 
     Although a non-contact type deflection mechanism and sensor is preferred, it is contemplated that other mechanisms may be employed. For instance, a contact type mechanical probe and associated sensor is shown as an alternative embodiment in FIG.  6 . 
     The embodiment of FIG. 6 employs a platform  99  that substitutes for the platform  47  described above. No vacuum pressure is applied to this platform. Rather, a single channel  101  is formed in the upper surface  103  of the platform. The paper  32  is directed across this surface  103 . An elongated probe  105  is normally suspended above the channel  101 . In one embodiment a ferromagnetic part  107  of the probe is held against an electromagnet  109 , that is turned on and off by the printer controller. When the electromagnet is tuned off (which occurs while the paper is beneath the probe), the probe  105  is released and moves toward the paper in a vertical path defined by annular guide members  111 . 
     The probe weight deflects the paper  32  as the tip of the released probe contacts it. The amount of paper deflection (relating to the overall distance that the probe travels) is measured as described next. 
     An optical sensor  100  measures the probe movement in deflecting the paper. The upper end  113  of the probe carries 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 probe end  113  (up and down in FIG.  6 ). As a result, the output from the sensor  100 , which is applied to the vacuum controller (the stiffness signal) varies with the probe movement, which, as described, correlates to a preferred vacuum pressure level to be applied to the drum interior. It will be appreciated that many other mechanical type sensors can be used to deflect the paper and quantify the deflection in a manner such as just described. 
     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 or channels in the drum as compared to what is depicted in the drawings. 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.