Abstract:
A method is disclosed for picking media in inkjet printers using a non-linear system, so that only a single sheet of media is drawn into the printer for printing. Inkjet printers operate by drawing media into the printer for printing. Two coil springs, arranged in parallel, support the pressure plate to bring media stack in contact with the pick roller. At full media stack, both coils are compressed; at low media stack, only one spring is compressed, the other is uncompressed, thus providing sufficient contact pressure between media and pick roller.

Description:
FIELD OF INVENTION  
         [0001]    The present invention relates generally to methods for handling printing media and more particularly for picking a single sheet from the media supply of a printer.  
         BACKGROUND AND SUMMARY OF INVENTION  
         [0002]    Media for printers includes paper and transparent film. The media is usually placed in a stack on the printer tray and drawn (picked) individually into the printer prior to printing. Normally, a single sheet of media is drawn into the printer for printing. If two or more sheets are picked, a “multiple pick” error results. If no sheet is picked, a “no pick” error results. Either case will cause the suspending of printing, wastage of media and the eventual frustration of the user.  
           [0003]    Usually in a printer, picking of media involves several elements: a stack of media, a printer tray, a coil spring, a pressure plate and a pick roller. A stack of media is placed on a printer tray. The leading edge of media rests between a pressure plate and a pick roller. The pressure plate supports the media stack and urges the media stack towards the pick roller just prior to picking. A coil spring beneath the pressure plate enables the pressure plate to provide varying supporting force at different stack height. When the pick mechanism is actuated, the pressure plate will move and lift up the media stack against the pick roller. The pick roller engages the media by friction means and rotates, thereby drawing media into the printer. The force with which the coil spring exerts on the pressure plate affects the contact pressure between the media and the pick roller. An excessive contact pressure results in multiple sheets being picked. A deficient contact pressure results in no contact and thereby no media being picked.  
           [0004]    Hence, proper media picking depends upon the coil spring. Conventional coil springs have not been able to consistently ensure adequate contact pressure so that only a single sheet of media is picked for printing. This is due to a presumption that as the height of the media stack increases, the force required to provide adequate contact pressure increases linearly. Observations show otherwise. As media stack height increases, the force required to provide adequate contact pressure increases non-linearly. This non-linear increase in force as media stack height increases will be referred to as a “non-linear” force. Due to the non-linear behavior, conventional coil spring system sometimes either under-compensates or over-compensates the required force and results in “no pick” or “multiple pick” errors respectively. If a spring stiffness is suitably selected for low media stack, the coil spring would under-compensate at high media stack and result in no pick at high media stack height. If a spring stiffness is suitably selected for high media stack, the coil spring would over-compensate at low media stack and result in multiple pick at low media stack height. The reliability of media pick is thus compromised.  
           [0005]    As there is a trend towards network printing, there is a need for significantly larger media capacity, and accordingly a need to devise reliable and inexpensive media pick apparatus.  
           [0006]    The present invention improves upon the prior art by having a non-linear force exerting on the pressure plate to compensate for the non-linear behavior as discussed. One embodiment employs two coil springs in parallel. This system overcomes the limitations of the prior art by providing a non-linear relationship between the force required to provide adequate contact pressure and the media stack height. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    [0007]FIG. 1 shows a simplified side view of a media pick apparatus at full media stack using a conventional one-spring system.  
         [0008]    [0008]FIG. 2 shows a simplified side view of a media pick apparatus at low media stack using a conventional one-spring system.  
         [0009]    [0009]FIG. 3 shows a linear relationship between the force exerted on a pressure plate and the height of a media stack (conventional one-spring system).  
         [0010]    [0010]FIG. 4 shows the characteristics of a linear spring system and a non-linear spring system.  
         [0011]    [0011]FIG. 5 shows a simplified side view of a printer tray at full media stack using a dual-spring system.  
         [0012]    [0012]FIG. 6 shows a simplified side view of a printer tray at low media stack using a dual-spring system.  
         [0013]    [0013]FIG. 7 shows the non-linear relationship between the force exerted on a pressure plate and the height of a media stack (dual-spring system).  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]    [0014]FIG. 1, FIG. 2 and FIG. 3 illustrate a prior art system.  
         [0015]    [0015]FIG. 1 shows a simplified side view of a media pick apparatus at full media stack  20   a  using a conventional one-spring system. The full media stack  20   a  can contain paper or transparent film or any material that the printer supports. The media stack  20   a  rests upon a pressure plate  11  which is supported by a coil spring  30   a  from below. The pressure plate  11  is fully horizontal at full stack height  20   a . The pick roller  10  rotates in a clockwise direction  13  and thereby draws via friction means a sheet of media along the trough  12  into the printer for printing.  
         [0016]    [0016]FIG. 2 shows a simplified side view of a media pick apparatus at low media stack  20   b  using a conventional one-spring system. The pressure plate  11  is normally inclined at an acute angle  14  to the horizontal plane.  
         [0017]    [0017]FIG. 3 shows a linear relationship between the force exerted on the pressure plate  11  by a conventional coil spring  30   a  and the height of media stack (conventional one-spring system). The slope of the graph is equivalent to the spring stiffness coefficient. The spring stiffness coefficient is a constant value characterized by the material of the spring. This coefficient is measured in Newtons per millimeter. The spring compression is computed as the displacement from the uncompressed length of the coil spring. The spring compression is measured in millimeter. At high stack height X1, the force required to provide adequate contact pressure between the media and the pick roller is F1. At low stack height X2, the force required to provide adequate contact pressure between the media and the pick roller is F2. Force F1 is greater than force F2, and a straight line can be drawn through the two points. The force exerted by coil spring on the pressure plate is computed as the product of the spring stiffness coefficient and the spring compression.  
         [0018]    [0018]FIG. 4 illustrates the characteristics of a linear spring system and a non-linear spring system. Curve  41  depicts the linear system. Curve  42  depicts the non-linear system. The characteristic of conventional spring system is described by Curve  41 . As the weight of the media stack increases, the force that is required to support the pressure plate  11  increases, so that adequate contact pressure between media and the pick roller  10  is maintained. Beyond a critical stack height X3, the force required is more than what is anticipated in a linear system. Hence, beyond the critical stack height X3, a stiffer spring is needed to provide for the larger force required. Accordingly, the spring stiffness coefficient (slope of the graph) has to increase. The characteristics of a non-linear spring system that will meet this requirement is accurately described by Curve  42 .  
         [0019]    There are several ways to implement a non-linear compressible system described by Curve  42 . One way is by arranging two coil springs of different length in parallel so that the spring stiffness coefficient of the whole system increases beyond a certain compression point X3. Below this point X3, only one coil spring is under compression and the spring stiffness coefficient of the whole system is the spring stiffness coefficient of the coil spring. Beyond this point X3, both coil springs are under compression and the spring stiffness coefficient of the system is the summation of both spring stiffness coefficients in the system.  
         [0020]    Another way to implement a non-linear compressible system is by using a piston containing gas or liquid. At higher levels of compression, the force needed per unit compression to compress the gas or liquid increases, thereby constituting a non-linear characteristic.  
         [0021]    Other ways to implement a non-linear compressible system include using a combination of springs of varying diameters and or thickness, a combination of spring or springs and piston or pistons, and the like.  
         [0022]    [0022]FIG. 5, FIG. 6 and FIG. 7 illustrate one embodiment of the present invention.  
         [0023]    In FIG. 5, there are two coil springs arranged in parallel supporting the pressure plate. A first coil spring  31  is longer and has a smaller diameter. A second coil spring  32  is shorter and has a wider diameter. The first coil spring  31  resides within the second coil spring  32 . Both springs are under compression due to the weight of a full media stack  20   a . Each spring exerts its respective force on the pressure plate  11 . As both springs are under compression, the spring stiffness coefficient of the whole system is the summation of all spring stiffness coefficients in the system.  
         [0024]    In FIG. 6, the first coil spring  31  is uncompressed whereas the second coil spring  32  is compressed. This phenomenon is due to a low media stack  20   b . Only the first coil spring  31  is exerting a force on the pressure plate  11 . The second coil spring  32 , being uncompressed, does not exert any force on the pressure plate  11 . As only the first coil spring  31  is compressed, the spring stiffness coefficient of the whole system is the spring stiffness coefficient of the first coil spring  31 .  
         [0025]    The operating procedure of a media pick apparatus with a dual-spring system works similarly to that of a conventional one-spring system. In a dual-spring system, the media stack rests upon a pressure plate  11  which is supported by a spring system from below. At full stack height  20   a , the pressure plate  11  is fully horizontal and is supported by the first  31  and second  32  coil springs. At low stack height  20   b , the pressure plate  11  is normally inclined at an acute angle  14  to the horizontal plane and is supported only by the first coil spring  31 . The pick roller  10  rotates in a clockwise direction  13  and thereby draws via friction means a sheet of media along the trough  12  into the printer for printing.  
         [0026]    [0026]FIG. 7 shows a non-linear relationship between the force exerted on the pressure plate and the height of the media stack in a dual-spring system. This relationship is described by Curve  42  in FIG. 4. At low media stack height  20   b , the force exerted must be low enough such that only one sheet of media is picked; otherwise, multiple sheets may be picked. At high media stack height  20   a , the force exerted must be high enough; otherwise, no sheet may be picked. As drawn in FIG. 7, the force F1′ exerted on the pressure plate in the dual-spring system is higher than the force F1 exerted on the pressure plate in the conventional one-spring system. The present non-linear spring system recognizes the non-linear behavior as discussed. Accordingly, the non-linear spring system provides the force required to keep the media in sufficient contact with the pick roller so that media is picked singly.  
         [0027]    From the foregoing analysis, the media pick reliability is improved by using a non-linear media pick system. The present invention ensures the consistent picking of only one sheet of media regardless of media stack height. While the illustrated system employs two coil springs arranged in parallel, the invention is not intended to be so limited.