[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.
[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.
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[0019] There are several ways to implement a non-linear compressible system described by Curve
[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.
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[0023] In
[0024] In
[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
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[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.