Abstract:
A positioning system for a carriage in a printing system includes a support plate having a groove along a length of the plate, and a rail positioned along the groove. A first adjusting mechanism is used to adjust the position of the rail in a first direction, and a second adjusting mechanism is used to adjust the position of the rail in a second direction that is substantially normal to the first direction.

Description:
RELATED APPLICATION  
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/244,299, filed on Oct. 30, 2000. The entire teachings of the above application are incorporated herein by reference. 
     
    
     
       BACKGROUND  
         [0002]    Certain types of printing systems are adapted for printing images on large-scale substrates, such as museum displays, billboards, sails, bus boards, and banners. Some of these systems use so-called drop on demand ink jet printing. In these systems, a piezoelectric vibrator applies pressure to an ink reservoir of the print head to force the ink out through the nozzle orifices positioned on the underside of the print heads. A particular image is created by controlling the order at which ink is ejected from the various nozzle orifices.  
           [0003]    In some of these systems, a carriage which holds a set of print heads scans across the width of a flexible substrate while the print heads deposit ink as the substrate moves. In another type of system, a solid, non-flexible substrate is supported on a table. The carriage holding the print heads has two degrees of motion so that it is able to move along the length as well as the width of the substrate as the print heads deposit ink onto the substrate. And in yet another arrangement, a solid, non-flexible substrate is held to a table as the entire table and substrate move together s along one axis of the substrate under the print heads as the carriage holding the print heads traverses in a direction normal to that axis while the print heads deposit ink to create a desired image.  
         SUMMARY  
         [0004]    During the printing process, as the carriage traverses along a pair of rails, the position of the carriage may vary because of the rails are not positioned to be precisely parallel to each other or within a plane. In some prior art systems, the rails are supported on a milled or machined support structure, or the rails are epoxied to a support structure. In these prior art systems, it is very difficult to readily adjust the position of the rails to within a desired tolerance. It is desirable, therefore, to adjust the position of the rails such that they are parallel to each other and are parallel relative to a common plane to within a desired tolerance.  
           [0005]    In one aspect of the invention, a positioning system for a carriage in a printing system includes a support plate having a groove along a length of the plate, and a rail positioned along the groove. A first adjusting mechanism is used to adjust the position of the rail in a first direction, and a second adjusting mechanism is used to adjust the position of the rail in a second direction that is substantially normal to the first direction.  
           [0006]    Embodiments of this aspect can include one or more of the following features. The groove can be shaped such that the rail makes a two-point contact with the groove along the length of the rail. The support plate has a second groove along the length of the plate located on an opposite side of the plate across the width of the plate. The first and the second groove are substantially parallel to each other. There can be a second rail positioned in the second groove. The position in the first direction can be maintained to a tolerance of about ±0.0005 inch, and the position in the second direction can be maintained to a tolerance of about ±0.0005 inch. The first and the second adjusting mechanisms can be jack-screw mechanisms.  
           [0007]    In a related aspect, a method of positioning a carriage holding a set of print heads includes adjusting the position of a rail aligned along a groove of a support plate in a first direction with one or more first adjusting mechanisms, and adjusting the position of the rail in a second direction that is substantially normal to the first position with one or more second adjusting mechanisms.  
           [0008]    The method can include adjusting the position of a second rail aligned along a second groove of the support plate that is substantially parallel to the first groove in the first direction with one or more of the first adjusting mechanisms. The first groove and the second groove can be located on opposite sides of the plate across the width of the plate. In some embodiments, the method includes adjusting the position of the second rail in the second direction with one or more of the second adjusting mechanisms. The adjusting mechanisms can be jack-screw mechanisms.  
           [0009]    Among other advantages, the present invention provides a cost-effective means for an operator of the positioning system to quickly align the rails with just two sets of adjusting mechanisms. Further, the rails can be presciely positioned within a desired tolerance. Even if a support beam to which the plates are secured sags, for example, in an unsupported midsection portion of the beam, an operator can easily compensate for this sag by adjusting the position of the rails so that they remain parallel to each other and to a common plane. Further, the combination of the rails and the support plates form a stiff truss. This truss structure is stable and dampens any motion imparted on the structure thereby minimizing any motion transmitted to the carriage, hence, minimizing any undesirable carriage motion. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.  
         [0011]    [0011]FIG. 1 is an perspective view of a printing system in accordance with the present invention.  
         [0012]    [0012]FIG. 2A is a cross-sectional and block diagram view of the printing system of FIG. 1.  
         [0013]    [0013]FIG. 2B is a top view of a transport belt of the printing system of FIG. 1.  
         [0014]    [0014]FIG. 3A is an isolated view of a thickness indicator roller of the printing system of FIG. 1.  
         [0015]    [0015]FIG. 3B is a side view of the thickness roller along the line  3 B- 3 B of FIG. 3A.  
         [0016]    [0016]FIG. 4A is an isometric view of a part of a rail system for supporting a carriage of the printing system of FIG. 1.  
         [0017]    [0017]FIG. 4B is a cross-sectional view of the rail system of FIG. 4A shown mounted to a support beam.  
         [0018]    [0018]FIG. 4C is a cross-sectional view of the rail system of FIG. 4A shown with the carriage of the printing system. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    A description of preferred embodiments of the invention follows.  
         [0020]    Referring to FIG. 1, there is shown a printing system  10  that prints on both flexible and non-flexible substrates. Further, the printing system  10  is able to accommodate substrates with various thickness automatically during the printing process.  
         [0021]    The printing system  10  includes a base  12 , a rail system  14  attached to the base  12 , a transport belt  18  which moves a substrate though the system, and a substrate thickness indicator roller  20 . A carriage  16  holding a set of print heads  17  (shown in phantom) is supported by and traverses along the rail system  14 .  
         [0022]    Referring further to FIG. 2, the set of print heads  17  which are typically positioned from about 0.04 inch to about 0.08 inch from a substrate  32  as the substrate moves through the system by the transport belt  18 . A carriage motor  48  such as, for example, a servo motor or any other suitable drive mechanism, of the carriage  16  is connected to a feedback device  50  and a carriage motor controller  52 . The motor controller  52  as well as the feedback device  50  transmit signals to a controller such as a central CPU  44 .  
         [0023]    As mentioned above, the printing system  10  is able to automatically accommodate changes in the thickness of the substrate. For example, if the thickness of the substrate increases or if the substrate is thicker than the previous substrate, as the substrate moves through the system, the indicator roller  20  which sits on top of the substrate rises. The increased thickness is detected in turn by a dial indicator  29  that is attached to the indicator roller  20 . This increased thickness information is transmitted from the dial indicator  29  to the CPU  44 . The CPU  44  then transmits a signal to the controller  52  to instruct the carriage motor  48  to move carriage  16  and hence the print heads  17  upwards away from the substrate. Meanwhile, the position of the carriage is relayed to the feedback device  50  and in turn to the CPU  44  which then determines if further finer adjustments are needed to position print heads  17  at the proper height. Thus regardless of the thickness and/or stiffness of the substrate, the printing system  10  maintains a precise desired gap between the print heads  17  and the substrate  32 . The printing system  10  is able to automatically accommodate a change in thickness of the substrate in about five seconds. In sum, the printing system  10  is capable of handling flexible substrates as well as solid non-flexing substrates with various thicknesses “on the fly” with minimal or no intervention from an operator.  
         [0024]    To prevent the substrate from slipping on the transport belt  18 , the printing system  10  also includes a vacuum table  22  provided with a set of holes  21 . A vacuum motor  42  supplies the vacuum to the vacuum table  22 , and the vacuum is detected by a vacuum sensor  40 . Both the vacuum sensor  40  and the vacuum motor  42  are connected to and under the direction of the CPU  44  which receives and transmits the appropriate signals to maintain the desired vacuum. In the illustrated embodiment, the vacuum provided by the vacuum table  22  is approximately in the range −0.05 psi to −0.3 psi.  
         [0025]    The transport belt  18  is provided with holes  100  (FIG. 2B) that extend through the thickness of the belt, each having a diameter of about 0.1 inch, which are spaced apart from one another by about one inch. The belt  18  is a woven polyester made from reinforced polyurethane, and has a thickness of about 0.09 inch. The woven polyester minimizes stretching of the belt  18  and thus provides high stepping accuracy and uniform vacuum distribution. Alternatively, the belt can be made from stainless steel having a thickness of about 0.008 inch.  
         [0026]    A porous sheet  43  having a thickness of about 0.5 inch sits between the vacuum table  22  and the transport belt  18 . The porous sheet is made from a sintered, porous polyethylene, or any other suitable material. The holes in the belt  18 , and the porous sheet  43  assure that a suction is applied to a substrate when a vacuum is provided by the vacuum table  22 . In essence, the porous sheet  43  acts as a flow resistor. Thus when the substrate covers only a portion of belt  18 , the vacuum provided by the vacuum table  22  does not have to be significantly readjusted, if at all, even as the area over the belt covered by the substrate varies. In sum, with the porous sheet  43 , a continuous vacuum can be provided by the vacuum table  22 , and no further adjustment to the vacuum level needs to be made as one or more substrates are transmitted through the printing system during the print process. This feature is applicable to both continuous substrates, for example, those supplied from a roll, as well as non-continuous substrates such as a flexible or a rigid sheet supplied individually.  
         [0027]    Turning now to the drive mechanism of the printing system  10 , the transport belt  18  wraps around a drive roller  24  and an idler roller  26 , while an optical encoder wheel  28  and the thickness indicator roller  20  sits on top the belt  18 . The idler roller  26  is able to move in the x-direction and through a dynamic tensioning device  29  keeps the belt  18  under a constant tension during the printing process.  
         [0028]    A drive motor  36  rotates the drive roller  24  which causes the belt  18  to move in the direction of arrow A, and is connected along with the encoder wheel  28  to a drive controller  38 . The encoder wheel  28  detects the precise distance that the substrate moves. This information is relayed to the drive controller  38  and in turn to the CPU  44 . The CPU  44  transmits a signal back to the controller  38  which controls the speed of the drive motor  36  so that the distance the substrate moves is precisely controlled. Thus the feedback position signals from the optical encoder  28  compensates for belt thickness variations, seams in the belt, and variations in the diameter of the rollers over time.  
         [0029]    In some embodiments, the feed wheel  30  supplies a flexible substrate  32 , which wraps underneath a dancer roller  34 , to the printing system. The feed wheel  30  is rotated by a feed motor  53  which is controlled by a feed controller  54 . Both the feed controller  54  and the dancer  34  are connected to a position sensor  55 , and located above and below the dancer  34  is a top limit switch  56   a  and a bottom limit switch  56   b , respectively.  
         [0030]    If during the printing process a jam occurs, the dancer  34  will rise and trigger the top switch  56   a  to send a signal to the central CPU  44  which then directs the printing system  10  to terminate the printing process because a problem has been detected. And if the feed roll  30  becomes depleted of the substrate material  32  during the printing process, the dancer  34  will drop down and trigger the bottom switch  56   b  to transmit a signal to the CPU  44  to shut the printing process off since there is no longer any substrate material.  
         [0031]    During the printing process, as the substrate  32  is fed by the feed wheel  30 , the position sensor  55  detects the height of the dancer  34 . This height information is transmitted to the feed controller  54  which in turn adjusts the power to the feed motor  53  to increase or reduce the feed speed, or to reverse the feed direction of feed wheel  30  such that a constant tension is maintained in the substrate. A constant tension is desired to maintain positional accuracy of the substrate and to remove any wrinkles in the substrate while it moves through the printing system.  
         [0032]    The printing system  10  can detect thickness variations of the substrate regardless of the width of the substrate or the position of the substrate relative to the width of belt  18 . This capability is illustrated in FIGS. 3A and 3B. As shown, the thickness indicator roller  20  rotates freely about a bar  21  that is supported by a pair of ratchet/gear mechanisms  57 , each of which includes a gear  58  engaged with a rachet  59 . Thus when a substrate causes the height of indicator roller  20  to vary, both of the gears  58  rotate so that the indicator roller  20  is at the same height, “h”, along the width, “w 1 ”, of the belt  818  regardless of the width, “w 2 ”, of the substrate  32  that is fed to the printer system. Note that the vertical position, “y” of the dancer  34  (FIG. 2) is also controlled by a similar ratchet/gear mechanism. Alternatively, a laser triangulation device is used to determine the thickness of the substrate.  
         [0033]    Referring now to FIGS. 4A and 4B, the rail system  14  includes a top rail  60  and a bottom rail  62 . These rails are attached to a set of spacer support plates  64  by a set of screws  65  along a bottom and a top machined V-groove  66   a  and  66   b , respectively. These grooves  66  provide a two-point contact with each of the rails  60  and  62 . This two-point contact is maintained along the entire length of the rails  60  and  62 . The set of support plates  64  is attached to a support beam  67  of the base  12  by a series of set screws  68 . The horizontal displacement, “x”, of the support plates  64  with respect to the support beam  66  is adjusted by a set of horizontal jack screws  70 . Each horizontal jack screw  70  is associated with a bellvile washer  71  that pushes the support plates  64  away from the support beam  66  to assure that the horizontal jack screws  70  are always under tension. The vertical position, “y”, of the support plates  64  is adjusted by a set of vertical jack screws  72 . The vertical jack screws  72  are threaded into a block  74  that is attached to the support beam  67 . The machined V-grooves  66 , and the jack screws  70  and  72  enable an operator to adjust the position of the rails  60  and  62  so that the rails remain parallel in a plane and parallel to one another to within a tolerance of about ±0.0005 inch which ensures the precise positioning of the print heads  17  relative to substrate.  
         [0034]    Also shown in FIGS. 4B and 4C is a pulley  76  and a carriage belt  78  that is attached to the carriage  16 . The pulley  76  and another similar pulley, one of which is connected to a motor, are located on either end of the rail system  14 . Referring in particular to FIG. 4C, the carriage  16  is provided with a set of sleeve bushings  80  to enable the carriage to slide along rails  60  and  62 . Accordingly, as the motor drives the pulley, the carriage  16  traverses partially or fully along the length of the rails  60  and  62 .  
         [0035]    In use, an operator activates the printer system  10  and places the substrate  32  onto the belt  18 . As mentioned above, the vacuum sensor  40  detects the vacuum of the vacuum table  22  as applied to the substrate  32 . This information is fed to the CPU  44  which controls the vacuum motor  42  to maintain the desired vacuum. Because porous sheet  43  acts as a flow resistor, large variations in the applied vacuum are not required. In fact, little or no variations in the applied vacuum are required in a typical printing process.  
         [0036]    The drive motor  36  rotates the drive roller  24  to move the transport belt  18  and hence the substrate  32  under the print heads  17 . Meanwhile, the dynamic tensioning device  29  of the idler roller  26  maintains a constant tension in the belt  18  during the printing operation. The translational movement of the substrate  32  underneath the print heads  17  is monitored by the encoder wheel  28  to ensure that this movement is precisely controlled.  
         [0037]    As the substrate moves under the carriage  16  and hence the print heads  17 , the carriage  16  traverses back and forth (that is, in and out of the page when referring to FIG. 2A) along the width of the substrate as instructed by the CPU  44 , while the print heads  17  deposit ink onto the substrate to create the desired image. The ink can be, for example, solvent pigment inks, UV resistant inks, or water inks. The through put of printing system  10  ranges from about 0.5 ft/min to about 10 ft/min.  
         [0038]    As discussed above, changes in the thickness of the substrate are automatically detected by the system. Thus, if a thin, flexible substrate is followed by a thicker, non-flexible substrate, the system automatically without the intervention of the operator adjusts the height of carriage  16  such that the proper gap is maintained between the print heads  17  and the substrate.  
         [0039]    While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.