DETAILED DESCRIPTION OF THE INVENTION

[0014] FIG. 1, FIG. 2 and FIG. 3 illustrate a prior art system.

[0015] FIG. 1 shows a simplified side view of a media pick apparatus at full media stack 20a using a conventional one-spring system. The full media stack 20a can contain paper or transparent film or any material that the printer supports. The media stack 20a rests upon a pressure plate 11 which is supported by a coil spring 30a from below. The pressure plate 11 is fully horizontal at full stack height 20a. 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] FIG. 2 shows a simplified side view of a media pick apparatus at low media stack 20b using a conventional one-spring system. The pressure plate 11 is normally inclined at an acute angle 14 to the horizontal plane.

[0017] FIG. 3 shows a linear relationship between the force exerted on the pressure plate 11 by a conventional coil spring 30a 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] 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] 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 20a. 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 20b. 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 20a, the pressure plate 11 is fully horizontal and is supported by the first 31 and second 32 coil springs. At low stack height 20b, 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] 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 20b, 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 20a, 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.